Resonant two-photon ionization spectroscopy of jet-cooled PdSi

The spectra of diatomic PdSi have been investigated for the first time, using the technique of resonant two-photon ionization spectroscopy. A number of vibronic transitions have been observed in the 20,400-22,000 cm(-1) range. It is difficult to group the bands into band systems, although one likely band system has been identified. Three bands have been rotationally resolved and analyzed, two of which are perturbed by interactions with other states. The data show that the ground state of PdSi has Ω = 0, and a bond length of r(0)(') = 2.0824(3) A?. Comparisons to previously published density functional theory calculations provide strong support for the assignment of the ground state to the 1σ(2) 2σ(2) 1π(4) 1δ(4) 3σ(2), (1)Σ(+) term, which is predicted to be the ground state in the calculations. The much shorter bond length and greater bond energy of PdSi, as compared to its isoelectronic counterpart, AlAg, demonstrate that there is strong π bonding in PdSi, as has been previously found for the other nickel group silicides, NiSi and PtSi.     

Electronic spectroscopy and electronic structure of diatomic TiFe

Diatomic TiFe, a 12 valence electron molecule that is isoelectronic with Cr(2), has been spectroscopically investigated for the first time. In addition, the first computational study that includes the ground and excited electronic states is reported. Like Cr(2), TiFe has a (1)Σ(+) ground state that is dominated by the 1σ(2) 2σ(2) 1π(4) 1δ(4) configuration. Rotationally resolved spectroscopy has established a ground state bond length of 1.7024(3) A?, quite similar to that found for Cr(2) (r(0) = 1.6858 A?). Evidently, TiFe exhibits a high degree of multiple bonding. The vibronic spectrum is highly congested and intense to the blue of 20?000 cm(-1), while two extremely weak band systems, the [15.9](3)Π(1) ← X (1)Σ(+) and [16.2](3)Π(0+) ← X (1)Σ(+) systems, are found in the 16?000-18?500 cm(-1) region. The bond lengths, obtained by inversion of the B(e) (') values, and vibrational frequencies of the two upper states are nearly identical: 1.886?A? and 344 cm(-1) for [15.9](3)Π(1) and 1.884 A? and 349 cm(-1) for [16.2](3)Π(0+). The measured spin-orbit splitting of the (3)Π state is consistent with its assignment to the 1σ(2) 2σ(2) 1π(4) 1δ(3) 2π(1) configuration, as is also found in the ab initio calculations.     

Absolute cross sections for electronic excitations of cytosine by low energy electron impact

The absolute cross sections (CSs) for electronic excitations of cytosine by electron impact between 5 and 18 eV were measured by electron-energy-loss (EEL) spectroscopy of the molecule deposited at low coverage on an inert Ar substrate. The lowest EEL features found at 3.55 and 4.02 eV are ascribed to transitions from the ground state to the two lowest triplet 1?(3)A(')(π→π(?)) and 2?(3)A(')(π→π(?)) valence states of the molecule. Their energy dependent CSs exhibit essentially a common maximum at about 6 eV with a value of 1.84×10(-17)?cm(2) for the former and 4.94×10(-17)?cm(2) for the latter. In contrast, the CS for the next EEL feature at 4.65 eV, which is ascribed to the optically allowed transition to the 2?(1)A(')(π→π(?)) valence state, shows only a steep rise to about 1.04×10(-16)?cm(2) followed by a monotonous decrease with the incident electron energy. The higher EEL features at 5.39, 6.18, 6.83, and 7.55 eV are assigned to the excitations of the 3?(3,1)A(')(π→π(?)), 4?(1)A(')(π→π(?)), 5?(1)A(')(π→π(?)), and 6?(1)A(')(π→π(?)) valence states, respectively. The CSs for the 3?(3,1)A(') and 4?(1)A(') states exhibit a common enhancement at about 10 eV superimposed on a more or less a steep rise, reaching, respectively, a maximum of 1.27 and 1.79×10(-16)?cm(2), followed by a monotonous decrease. This latter enhancement and the maximum seen at about 6 eV in the lowest triplet states correspond to the core-excited electron resonances that have been found by dissociative electron attachment experiments with cytosine in the gas phase. The weak EEL feature found at 5.01 eV with a maximum CS of 3.8×10(-18)?cm(2) near its excitation threshold is attributed to transitions from the ground state to the 1?(3,1)A(")(n→π(?)) states. The monotonous rise of the EEL signal above 8 eV is attributed to the ionization of the molecule. It is partitioned into four excitation energy regions at about 8.55, 9.21, 9.83, and 11.53 eV, which correspond closely to the ionization energies of the four highest occupied molecular orbitals of cytosine. The sum of the ionization CS for these four excitation regions reaches a maximum of 8.1×10(-16)?cm(2) at the incident energy of 13 eV.     

Spectroscopic investigations of ThF and ThF+

The electronic spectra of ThF and ThF(+) have been examined using laser induced fluorescence and resonant two-photon ionization techniques. The results from high-level ab initio calculations have been used to guide the assignment of these data. Spectra for ThF show that the molecule has an X (2)Δ(3/2) ground state. The upper spin-orbit component, X (2)Δ(5/2) was found at an energy of 2575(15) cm(-1). The low-lying states of ThF(+) were probed using dispersed fluorescence and pulsed field ionization-zero kinetic energy (PFI-ZEKE) photoelectron spectroscopy. Vibronic progressions belonging to four electronic states were identified. The lowest energy states were clearly (1)Σ(+) and (3)Δ(1). Although the energy ordering could not be rigorously determined, the evidence favors assignment of (1)Σ(+) as the ground state. The (3)Δ(1) state, of interest for investigation of the electron electric dipole moment, is just 315.0(5) cm(-1) above the ground state. The PFI-ZEKE measurements for ThF yielded an ionization energy of 51 581(3) cm(-1). Molecular constants show that the vibrational constant increases and the bond length shortens on ionization. This is consistent with removal of a non-bonding Th-centered 6d or 7s electron. Laser excitation of ThF(+) was used to probe electronically excited states in the range of 19,000-21,500 cm(-1).     

Role of higher excited electronic states on high harmonic generation in H2(+)--a time-independent Hermitian Floquet approach

We have theoretically studied the role of high-lying molecular electronic states on the high harmonic generation (HHG) in H(2)(+) within the framework of a time-independent Hermitian nonperturbative three-dimensional Floquet technique for continuous wave monochromatic lasers of intensities of 2.59 × 10(13), 4.0 × 10(13), and 5.6 × 10(13) W∕cm(2), and wavelengths of 1064, 532, and 355 nm. To evaluate the HHG spectra, the resonance Floquet quasienergy and the Fourier components of the Floquet state corresponding to the initial vibrational-rotational level v = 0, J = 0 have been computed by solving the time-independent close-coupled Schro?dinger equation following the Floquet method. The calculations include seven molecular electronic states in the basis set expansion of the Floquet state. The electronic states considered, apart from the two lowest 1sσ(g) and 2pσ(u) states, are 2pπ(u), 2sσ(g), 3pσ(u), 3dσ(g), and 4fσ(u). All the concerned higher excited molecular electronic states asymptotically degenerate into the atomic state H(2 l) with l = 0, 1. The computations reveal signature of significant oscillations in the HHG spectra due to the interference effect of the higher molecular electronic states for all the considered laser intensities and wavelengths. We have attempted to explain, without invoking any ionization, the dynamics of HHG in H(2)(+) within the framework of electronic transitions due to the electric dipole moments and the nuclear motions on the field coupled ground, the first and the higher excited electronic states of this one-electron molecular ion.     

Phenyloxenium ions: more like phenylnitrenium ions than isoelectronic phenylnitrenes?

The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph-O(+)) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration ((1)A(1)) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state ((3)A(2)). Open-shell singlet configurations ((1)A(2), (1)B(1), (1)B(2), 2(1)A(1)) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet-triplet energy gap (ΔE(ST)) is found to have a positive linear free energy relationship with the Hammett-like σ(+)(R)/σ(+) substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and p-cyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have low-energy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar-NH(+)) than their isoelectronic arylnitrene (Ar-N) counterparts.     

Photoelectron imaging spectroscopy and theoretical investigation of ZrSi

The photoelectron spectrum of ZrSi(-) has been measured at two different photon energies: 2.33 eV and 3.49 eV, providing electron binding energy and photoelectron angular distribution information. The obtained vertical detachment energy of ZrSi(-) is 1.584(14) eV. The neutral ground and excited state terms are assigned based on experimental and theoretical results. The ground state of ZrSi is tentatively assigned as a (3)Σ(+) state with a configuration of 1σ(2) 1π(4) 1δ(0) 2σ(1) 3σ(1). A low lying (3)Π(i) neutral excited state is identified to be 0.238 eV (1919 cm(-1)) above the ground state. The anion ground state is designated as a (2)Σ(+) state with a 1σ(2) 1π(4) 1δ(0) 2σ(2) 3σ(1) valence electron configuration. A Franck-Condon (FC) simulation of the photoelectron spectrum has been carried out. For the (3)Σ(+) ← (2)Σ(+) band, theoretically calculated bond lengths and frequencies are used in the FC calculation which give good agreement with experiment, while for the (3)Π(i) ← (2)Σ(+) band, the ZrSi bond length is estimated from the FC spectrum. Comparisons are made with previously published theoretical studies and inconsistencies are pointed out. To the best of our knowledge, this study provides the first spectroscopic information on the transition metal-silicon diatomic, ZrSi.     

Time-resolved study of excited states of N2 near its first ionization threshold

Two-photon, two-color double-resonance ionization spectroscopy combining synchrotron vacuum ultraviolet radiation with a tunable near-infrared (NIR) laser has been used to investigate gerade symmetry states of the nitrogen molecule. The rotationally resolved spectrum of an autoionizing (1)Σ(g)(-) state has been excited via the intermediate c(4) (v = 0) (1)Π(u) Rydberg state. We present the analysis of the band located at T(v) = 10,800.7 ± 2 cm(-1) with respect to the intermediate state, 126,366 ± 11 cm(-1) with respect to the ground state, approximately 700 cm(-1) above the first ionization threshold. From the analysis a rotational constant of B(v) = 1.700 ± 0.005 cm(-1) has been determined for this band. Making use of the pulsed structure of the two radiation beams, lifetimes of several rotational levels of the intermediate state have been measured. We also report rotationally-averaged fluorescence lifetimes (300 K) of several excited electronic states accessible from the ground state by absorption of one photon in the range of 13.85-14.9 eV. The averaged lifetimes of the c(4) (0) and c(5) (0) states are 5.6 and 4.4 ns, respectively, while the b(') (12), c(')(4) (4, 5, 6), and c(')(5) (0) states all have lifetimes in the range of hundreds of picoseconds.     

1 Pi<--X1 Sigma+ band systems of jet-cooled ScCo and YCo

Rotationally resolved resonant two-photon ionization (R2PI) spectra of ScCo and YCo are reported. The measured spectra reveal that these molecules possess ground electronic states of (1)Sigma(+) symmetry, as previously found in the isoelectronic Cr(2) and CrMo molecules. The ground state rotational constants for ScCo and YCo are B(0)(")=0.201 31(22) cm(-1) and B(0) (")=0.120 96(10) cm(-1), corresponding to ground state bond lengths of r(0) (")=1.812 1(10) A and r(0) (")=1.983 0(8) A, respectively. A single electronic band system, assigned as a (1)Pi<--X (1)Sigma(+) transition, has been identified in both molecules. In ScCo, the (1)Pi state is characterized by T(0)=15,428.8, omega(e)(')=246.7, and omega(e)(')x(e)(')=0.73 cm(-1). In YCo, the (1)Pi state has T(0)=13 951.3, omega(e)(')=231.3, and omega(e)(')x(e) (')=2.27 cm(-1). For YCo, hot bands originating from levels up to v(")=3 are observed, allowing the ground state vibrational constants omega(e)(")=369.8, omega(e)(")x(e)(")=1.47, and Delta G(12)(")=365.7 cm(-1) to be deduced. The bond energy of ScCo has been measured as 2.45 eV from the onset of predissociation in a congested vibronic spectrum. A comparison of the chemical bonding in these molecules to related molecules is presented.     

塞曼效应在Cl2+及其同位素分子离子光谱标识中的应用

The A2Πu-X2Πg electronic absorption spectrum of the Cl2+ molecular cation in the region between 16820 and 17350 cm-1 was observed by employing optical heterodyne magnetic rotation enhanced velocity modulation spectroscopy. Cl2+ is a paramagnetic molecule; however, the intensities of some spectral lines, belonging to three bands whose origins are near 17282, 17324 and 16913 cm-1, respectively, remain unchanged with in the magnetic field. This indicates that both the upper and lower states have a weak Zeeman effect. The Zeeman contribution is nearly zero for the 2Π1/2 state, while nonvanishing for the 2Π3/2 state. Therefore, this behavior for the spectral assignment of Cl2+, including its isotopics was utilized and the identity of these bands was confirmed as members of the Ω=1/2 component of the electronic transition conveniently and unambiguously. The assigned bands are the (3, 7) band of the Ω=1/2 component of 35Cl+2 and 35Cl37Cl+ and the (2, 7) band of the Ω=1/2 component of 35Cl2+. It extends the range of vibrational assignments considerably in both the ground and the excited state, and leads to the successful rotational analysis. New molecular constants of Cl2+ were obtained from the observed line positions, band by band, using a weighted least squares fitting procedure.     

Photodissociation studies of the electronic and vibrational spectroscopy of Ni(+)(H2O)

The electronic spectrum of Ni?(H?O) has been measured from 16200 to 18000 cm?1 using photofragment spectroscopy. Transitions to two excited electronic states are observed; they are sufficiently long-lived that the spectrum is vibrationally and partially rotationally resolved. An extended progression in the metal-ligand stretch is observed, and the absolute vibrational quantum numbering is assigned by comparing isotopic shifts between ??Ni?(H?O) and ??Ni?(H?O). Time-dependent density functional calculations aid in assigning the spectrum. Two electronic transitions are observed, from the 2A? ground state (which correlates to the 2D, 3d? ground state of Ni?) to the 32A? and 22A? excited states. These states are nearly degenerate and correlate to the 2F, 3d?4s excited state of Ni?. Both transitions are quite weak, but surprisingly, the transition to the 2A? state is stronger, although it is symmetry-forbidden. The 3d?4s states of Ni? interact less strongly with water than does the ground state; therefore, the excited states observed are less tightly bound and have a longer metal-ligand bond than the ground state. Calculations at the CCSD(T)/aug-cc-pVTZ level predict that binding to Ni? increases the H-O-H angle in water from 104.2 to 107.5° as the metal removes electron density from the oxygen lone pairs. The photodissociation spectrum shows well-resolved rotational structure due to rotation about the Ni-O axis. This permits determination of the spin rotation constants ε(αα)' = -12 cm?1 and ε(αα)' = -3 cm?1 and the excited state rotational constant A' = 14.5 cm?1. This implies a H-O-H angle of 104 ± 1° in the 22A? excited state. The O-H stretching frequencies of the ground state of Ni?(H?O) were measured by combining IR excitation with visible photodissociation in a double resonance experiment. The O-H symmetric stretch is ν?' = 3616.5 cm?1; the antisymmetric stretch is ν?' = 3688 cm?1. These values are 40 and 68 cm?1 lower, respectively, than those in bare H?O.     

Low-lying excited states and nonradiative processes of the adenine analogues 7H- and 9H-2-aminopurine

We have investigated the UV vibronic spectra and excited-state nonradiative processes of the 7H- and 9H-tautomers of jet-cooled 2-aminopurine (2AP) and of the 9H-2AP-d(4) and -d(5) isotopomers, using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.045 cm(-1) resolution. The S(1) ← S(0) transition of 7H-2AP was observed for the first time. It lies ～1600 cm(-1) below that of 9H-2AP, is ～1000 times weaker and exhibits only in-plane vibronic excitations. In contrast, the S(1) ← S(0) spectra of 9H-2AP, 9H-2AP-d(4), and 9H-2AP-d(5) show numerous low-frequency bands that can be systematically assigned to overtone and combinations of the out-of-plane vibrations ν(1)', ν(2)', and ν(3)'. The intensity of these out-of-plane bands reflects an out-of-plane deformation in the (1)ππ?(L(a)) state. Approximate second-order coupled-cluster theory also predicts that 2-aminopurine undergoes a "butterfly" deformation in its lowest (1)ππ? state. The rotational contours of the 9H-2AP, 9H-2AP-d(4), and 9H-2AP-d(5) 0(0)(0) bands and of eight vibronic bands of 9H-2AP up to 0(0)(0) + 600 cm(-1) exhibit 75%-80% in-plane (a∕b) polarization, which is characteristic for a (1)ππ? excitation. A 20%-25% c-axis (perpendicular) transition dipole moment component may indicate coupling of the (1)ππ? bright state to the close-lying (1)nπ? dark state. However, no (1)nπ? vibronic bands were detected below or up to 500 cm(-1) above the (1)ππ? 0(0)(0) band. Following (1)ππ? excitation, 9H-2AP undergoes a rapid nonradiative transition to a lower-lying long-lived state with a lifetime ≥5 μs. The ionization potential of 9H-2AP was measured via the (1)ππ? state (IP = 8.020 eV) and the long-lived state (IP > 9.10 eV). The difference shows that the long-lived state lies ≥1.08 eV below the (1)ππ? state. Time-dependent B3LYP calculations predict the (3)ππ? (T(1)) state 1.12 eV below the (1)ππ? state, but place the (1)nπ? (S(1)) state close to the (1)ππ? state, implying that the long-lived state is the lowest triplet (T(1)) and not the (1)nπ? state.     

Photoinitiated predissociation of the NO dimer in the region of the second and third NO stretch overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.     

Experimental and theoretical study of the electronic spectrum of BeAl

The electronic structure of BeAl was investigated by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. BeAl was formed by pulsed laser ablation of a Be/Al alloy in the presence of helium carrier gas, followed by a free jet expansion into vacuum. In agreement with recent ab initio studies, the molecule was found to have a (2)Pi(1/2) ground state. Transitions to two low lying electronic states, (2)(2)Pi(1/2)(v') <-- X (2)Pi(1/2) (v' = 0) and (1)(2)Delta(v') <-- X (2)Pi(1/2) (v' = 0,1), were observed and rotationally analyzed. An additional band system, identified as (4)(2)Sigma(+)(v') <-- X (2)Pi(1/2), was found in the 28 000-30 100 cm(-1) energy range. This transition exhibited an unusual pattern of vibrational levels resulting from an avoided crossing with the (5)(2)Sigma(+) electronic state. New multi-reference configuration interaction calculations were carried out to facilitate the interpretation of the UV bands.An ionization energy of 48 124(80) cm(-1) was determined for BeAl from photoionization efficiency (PIE) measurements. Fine structure in the PIE curve was attributed to resonances with Rydberg series correlating with vibrationally excited states of the BeAl(+) ion. Analysis of this structure yielded a vibrational frequency of 240(20) cm(-1) for the cation.     

Gas phase electronic spectrum of linear AlCCH

The electronic spectrum of the aluminium containing species AlCCH has been detected in the gas phase in the region 315-355 nm. The experiment used a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Structures of the AlCCH isomers have been optimized using density functional theory (DFT) and the excitation energies to the low-lying electronic excited states calculated. Based on the analysis of the observed rotational structure and the theoretical data, the spectrum is assigned to the A (1)Pi<-- X (1)Sigma(+) electronic transition of linear AlCCH. The vibronic band system is complicated by the Renner-Teller effect in the excited state. The assignment yields nu(4)' = 516.4 cm(-1) for the stretching mode in the ground X (1)Sigma(+) state and nu(4)' = 654.5 cm(-1) for A (1)Pi excited state. Molecular constants determined from the rotational analysis are B(0)' = 0.16487(14), B(0)' = 0.17845(13) and T(0) = 28 755.04 cm(-1). The experimental and theoretical data indicate a shorter Al-C bond in the A (1)Pi excited than the X (1)Sigma(+) ground state.     

Optical spectroscopy of RuC: 18,000-24,000 cm(-1)

The optical spectrum of diatomic RuC has been recorded from 17 800 to 24 200 cm(-1). Three previously unidentified excited electronic states were analyzed and identified as having Omega' = 0, Omega' = 2, and Omega' = 3. The Omega' = 3 state was determined to be a 3Delta3 state that is suggested to arise from a mixture of the 10sigma(2)11sigma(2)5pi(3)2delta(3)12sigma(1)6pi(1) and 10sigma(2)11sigma(1)5pi(3)2delta(3)12sigma(2)6pi(1) electronic configurations. Three additional bands belonging to the previously observed [18.1] (1)Pi<--X (1)Sigma(+) system were analyzed to obtain B(e) (')=0.558 244(48) cm(-1), alpha(e) (')=0.004 655(27) cm(-1), omegae' = 887.201(37) cm(-1), and omega(e) 'xe' = 5.589(7) cm(-1) for the 102Ru 12C isotopomer (1sigma error limits). A Rydberg-Klein-Rees analysis was then performed using the determined spectroscopic constants of the [18.1] 1Pi state, and similar analyses were performed for the previously observed states. The resulting potential energy curves are provided for the 100Ru 12C, 101Ru 12C, 102Ru 12C, and 104Ru 12C isotopic species.     

S0 and S1 state structure, methyl torsional barrier heights, and fast intersystem crossing dynamics of 5-methyl-2-hydroxypyrimidine

We report the analysis of the S1<--S0 rotational band contours of jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. Unlike thymine, which exhibits a structureless spectrum, the vibronic spectrum of 5M2HP is well structured, allowing us to determine the rotational constants and the methyl group torsional barriers in the S0 and S1 states. The 0(0)(0), 6a(0)(1), 6b(0)(1), and 14(0)(1) band contours were measured at 900 MHz (0.03 cm(-1)) resolution using mass-specific two-color resonant two-photon ionization (2C-R2PI) spectroscopy. All four bands are polarized perpendicular to the pyrimidine plane (>90% c type), identifying the S1<--S0 excitation of 5M2HP as a 1nπ* transition. All contours exhibit two methyl rotor subbands that arise from the lowest 5-methyl torsional states 0A" and 1E". The S0 and S1 state torsional barriers were extracted from fits to the torsional subbands. The 3-fold barriers are V3" = 13 cm(-1) and V3' = 51 cm(-1); the 6-fold barrier contributions V6" and V6' are in the range of 2-3 cm(-1) and are positive in both states. The changes of A, B, and C rotational constants upon S1 <--S0 excitation were extracted from the contours and reflect an “anti-quinoidal” distortion. The 0(0)(0) contour can only be simulated if a 3 GHz Lorentzian line shape is included, which implies that the S1(1nπ*) lifetime is ~55 ps. For the 6a(0)(1) and 6b(0)(1) bands, the Lorentzian component increases to 5.5 GHz, reflecting a lifetime decrease to ~30 ps. The short lifetimes are consistent with the absence of fluorescence from the 1nπ* state. Combining these measurements with the previous observation of efficient intersystem crossing (ISC) from the S1 state to a long-lived T1 (3nπ*) state that lies ~2200 cm(-1) below [S. Lobsiger, S. et al. Phys. Chem. Chem. Phys. 2010, 12, 5032] implies that the broadening arises from fast intersystem crossing with k(ISC) ≈ 2 × 10(10) s(-1). In comparison to 5-methylpyrimidine, the ISC rate is enhanced by at least 10 000 by the additional hydroxy group in position 2.     

A combined spectroscopic and computational study of a high-spin S = 7/2 diiron complex with a short iron-iron bond

The nature of the iron-iron bond in the mixed-valent diiron tris(diphenylforamidinate) complex Fe(2)(DPhF)(3), which was first reported by Cotton, Murillo et al. (Inorg. Chim. Acta 1994, 219, 7-10), has been examined using additional spectroscopic and theoretical methods. It is shown that the coupling between the two iron centers is strongly ferromagnetic, giving rise to an octet spin ground state. On the basis of M?ssbauer spectroscopy, the two iron centers, formally mixed-valent Fe(II)Fe(I), are completely equivalent with an isomer shift δ = 0.65 mm s(-1) and quadrupole splitting ΔE(Q) = +0.32 mm s(-1). A large, positive zero-field splitting D(7/2) = 8.2 cm(-1) has been determined from magnetic susceptibility measurements. Multiconfigurational quantum studies of the complete molecule Fe(2)(DPhF)(3) found one dominant configuration (σ)(2)(π)(4)(π*)(2)(σ*)(1)(δ)(2)(δ*)(2), which accounts for 73% of the ground-state wave function. By considering all the configurations, an estimated metal-metal bond order of 1.15 has been calculated. Finally, Fe(2)(DPhF)(3) exhibits weak electronic absorptions in the visible and near-infrared regions, which are assigned as d-d transitions from the doubly occupied metal-metal π molecular orbital to half-occupied π*, δ, and δ* orbitals.     

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: two-dimensional resonance enhanced multiphoton ionization of HBr via singlet-, triplet-, Ω = 0 and 2 states

Mass spectra were recorded for one-colour resonance enhanced multiphoton ionization (REMPI) of H(i)Br (i = 79, 81) for the two-photon resonance excitation region 79,040-80,300 cm(-1) to obtain two-dimensional REMPI data. The data were analysed in terms of rotational line positions, intensities, and line-widths. Quantitative analysis of the data relevant to near-resonance interactions between the F(1)Δ(2)(v' = 1) and V(1)Σ(+)(v' = m + 7) states gives interaction strengths, fractional state mixing, and parameters relevant to dissociation of the F state. Qualitative analysis further reveals the nature of state interactions between ion-pair states and the E(1)Σ(+) (v' = 1) and H(1)Σ(+)(v' = 0) Rydberg states in terms of relative strengths and J' dependences. Large variety in line-widths, depending on electronic states and J' quantum numbers, is indicative of number of different predissociation channels. The relationship between line-widths, line-shifts, and signal intensities reveals dissociation mechanisms involving ion-pair to Rydberg state interactions prior to direct or indirect predissociations of Rydberg states. Quantum interference effects are found to be important. Moreover, observed bromine atom (2 + 1) REMPI signals support the importance of Rydberg state predissociation channels. A band system, not previously observed in REMPI, was observed and assigned to the k(3)Π(0)(v' = 0) ←← X transition with band origin 80,038 cm(-1) and rotational parameter B(v('))=7.238 cm(-1).     

The electronic spectrum of Si3 I: triplet D(3h) system

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si(3) was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm(-1) region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1(3)A(1)" - a? (3)A(2)' transition of the D(3h) isomer. The spectrum has an origin transition at 18,600 ± 4 cm(-1) and a short progression in the symmetric stretch with a frequency of ～445 cm(-1), in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1(3)A(1)" state, which has a calculated symmetric stretching frequency of 480 cm(-1). In addition, a ～505 cm(-1) ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D(3h) state and with theory. A weaker, overlapping band system with a ～360 cm(-1) progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C(2v) ground state ((1)A(1)) state of Si(3). Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.