Ultraviolet spectroscopy of pyrene in a supersonic jet and in liquid helium droplets

In a series of experiments devoted to the study of polycyclic aromatic hydrocarbons for astrophysical applications, the S(2)<--S(0) transition of jet-cooled pyrene (C(16)H(10)) at 321 nm has been studied by an absorption technique for the first time. The spectra observed by cavity ring-down spectroscopy closely resemble the excitation spectra obtained earlier by laser-induced fluorescence (LIF) and show the same band clusters arising from the vibronic interaction of S(2) with S(1). We have also investigated pyrene when it was incorporated into 380 mK cold helium droplets. These spectra which were recorded employing LIF and molecular beam depletion spectroscopy are broadened and redshifted by 0.94 nm but retain the essential features of the gas phase spectra.     

The visible spectrum of zirconium dioxide, ZrO2

The electronic spectrum of a cold molecular beam of zirconium dioxide, ZrO(2), has been investigated using laser induced fluorescence (LIF) in the region from 17,000 cm(-1) to 18,800 cm(-1) and by mass-resolved resonance enhanced multi-photon ionization (REMPI) spectroscopy from 17,000 cm(-1)-21,000 cm(-1). The LIF and REMPI spectra are assigned to progressions in the A?(1)B(2)(ν(1), ν(2), ν(3)) ← X?(1)A(1)(0, 0, 0) transitions. Dispersed fluorescence from 13 bands was recorded and analyzed to produce harmonic vibrational parameters for the X?(1)A(1) state of ω(1) = 898(1) cm(-1), ω(2) = 287(2) cm(-1), and ω(3) = 808(3) cm(-1). The observed transition frequencies of 45 bands in the LIF and REMPI spectra produce origin and harmonic vibrational parameters for the A?(1)B(2) state of T(e) = 16,307(8) cm(-1), ω(1) = 819(3) cm(-1), ω(2) = 149(3) cm(-1), and ω(3) = 518(4) cm(-1). The spectra were modeled using a normal coordinate analysis and Franck-Condon factor predictions. The structures, harmonic vibrational frequencies, and the potential energies as a function of bending angle for the A?(1)B(2) and X?(1)A(1) states are predicted using time-dependent density functional theory, complete active space self-consistent field, and related first-principle calculations. A comparison with isovalent TiO(2) is made.     

Insights on the CN B 2Σ+ + Ar potential from ultraviolet fluorescence excitation and infrared depletion studies of the CN-Ar complex

UV laser-induced fluorescence and IR-UV fluorescence depletion studies have been used to characterize the intermolecular levels of the CN-Ar complex in the excited state correlating with CN B (2)Σ(+) + Ar. Additional CN-Ar features are identified to lower wavenumber than reported previously. Fluorescence depletion spectra are recorded to confirm that these CN-Ar features and other higher energy features in the B-X spectrum originate from a common ground state level. The UV depletion is induced by IR excitation of CN-Ar from the ground state zero-point level to a hindered internal rotor state (n(K) = 1(1)) in the CN overtone region. The lowest energy feature in the B-X spectrum at 25,714.1 cm(-1) is assigned as a transition to the zero-point level of the B state and also yields its binding energy, D(0) = 186(2) cm(-1), which is in excellent accord with theoretical predictions. The next feature approximately 40 cm(-1) higher is attributed to overlapping transitions to intermolecular levels with bend (v(b)(K)=1(1)) or stretch (v(s) = 1) excitation. Yet higher features (previously reported) are also assigned, based on their transition type and wavenumber, which are consistent with the intermolecular energy level pattern computed theoretically. Finally, the intensity profile of the lowest energy features in the B-X spectrum reflects the predicted change in the CN (B (2)Σ(+), X (2)Σ(+)) + Ar potentials upon electronic excitation from a weakly anisotropic potential about the linear N≡C-Ar configuration in the ground state to a more strongly bound linear C≡N-Ar structure in the excited B electronic state.     

Triplet state excitation of alkali molecules on helium droplets: experiments and theory

In this paper, we discuss the electronic structure of alkali dimer molecules in 3Pig states on the surface of a helium droplet. The perturbation due to the droplet will in general not satisfy rotational symmetry around the internuclear axis of the diatom and thus, in addition to a broadening and blue shift, will cause a splitting of electronic levels that are degenerate in the free molecules. We propose a model based on general symmetry arguments and on a small number of physically reasonable parameters. We demonstrate that such a model accounts for the essential features of laser-induced fluorescence (LIF) and magnetic circular dichroism (MCD) spectra of the (1)3Pig-a3Sigma+ transition of Rb2 and K2. Furthermore the MCD spectra, analyzed according to the approach of Langford and Williamson [J. Phys. Chem. A 1998, 102, 2415], allow a determination of the populations of Zeeman sublevels in the ground state and thus a measurement of the surface temperature of the droplet. The latter agrees with the accepted temperature, 0.37 K, measured in the interior of a droplet.     

Ab initio calculations and Franck-Condon simulation of the absorption spectra of GeCl2 including anharmonicity.

Geometrical parameters, vibrational frequencies and relative electronic energies of the X1A1, ?3B1 and A1B1 states of GeCl2 have been calculated at the CCSD(T) and/or CASSCF/MRCI level with basis sets of up to aug-cc-pV5Z quality. Core electron correlation and relativistic contributions were also investigated. RCCSD(T)/ aug-cc-pVQZ potential energy functions (PEFs) of the X1A1 and ?3B, states, and a CASSCF/MRCl/aug-cc-pVQZ PEF of the A1B1 state of GeCl2 are reported. Anharmonic vibrational wavefunctions of these electronic states of GeCl2, obtained variationally using the computed PEFs, are employed to calculate the Franck-Condon factors (FCFs) of the ?-X and A-X transitions of GeCl2. Simulated absorption spectra of these transitions based on the computed FCFs are compared with the corresponding experimental laser-induced fluorescence (LIF) spectra of Karolczak et al. [J. Chem. Phys. 1993, 98, 60-70]. Excellent agreement is obtained between the simulated absorption spectrum and observed LIF spectrum of the ?-X transition of GeCl2, which confirms the molecular carrier, the electronic states involved and the vibrational assignments of the LIF spectrum. However, comparison between the simulated absorption spectrum and experimental LIF spectrum of the A-X transition of GeCl2 leads to a revision of vibrational assignments of the LIF spectrum and suggests that the X1A1 state of GeCl2 was prepared in the experimental work, with a non-Boltzmann vibrational population distribution. The X(0,0,1) level is populated over 4000 times more than expected from a Boltzmann distribution at 60 K, which is appropriate for the relative population of the other low-lying vibrational levels, such as the X(1,0,0) and X(0,1,0) levels.     

Fluorescence quenching in cyclic hydrogen-bonded complexes of 1H-pyrrolo[3,2-h]quinoline with methanol: cluster size effect

Laser induced fluorescence (LIF) excitation spectra of molecular complexes of 1H-pyrrolo[3,2-h]quinoline (PQ) with methanol (n= 1, 2, 3) as well as vibrational spectra of their electronic ground state are reported. The latter have been recorded in the mid-infrared region by fluorescence depletion (FDIR). The only PQ.methanol(n) complex clearly identified in the LIF spectrum is the triply hydrogen-bonded cyclic 1 ratio 2 aggregate. Its stoichiometry has been proven by the femtosecond multiphoton ionization detected infrared measurements [J. Am. Chem. Soc., 2006, 128, 10 000]. The structure of the 1 ratio 2 cluster is determined by means of FDIR spectroscopy in combination with ab initio and DFT calculations. No fluorescence was detected that could be attributed to the 1 ratio 1 cluster. This behaviour of the 1 ratio 1 complex is explained in terms of rapid excited state double proton transfer followed by a non-radiative relaxation. The n= 3 and heavier clusters are fluorescent. Their electronic spectra overlap, preventing the selective measurement of the FDIR spectra of individual complexes.     

Laser-induced fluorescence spectra of 4-methylcyclohexoxy radical and perdeuterated cyclohexoxy radical and direct kinetic studies of their reactions with O2

The laser-induced fluorescence (LIF) excitation spectra of the 4-methylcyclohexoxy and d11-cyclohexoxy radicals have been measured for the first time. LIF intensity was used as a probe in direct kinetic studies of the reaction of O(2) with trans-4-methylcyclohexoxy and d11-cyclohexoxy radicals from 228 to 301 K. Measured rate constants near room temperature are uniformly higher than the Arrhenius fit to the lower-temperature data, which can be explained by the regeneration of cyclic alkoxy radicals from the product of their beta-scission and the effect of O(2) concentration on the extent of regeneration. The Arrhenius expressions obtained over more limited ranges were k(O2) = (1.4(+8)(-1)) x 10(-13) exp[(-810 +/- 400)/T] cm(3) molecule(-1) s(-1) for trans-4-methylcyclohexoxy (228-292 K) and k(O2) = (3.7(+4)(-1)) x 10(-14) exp )[(-760 +/- 400) /T] cm(3) molecule(-1) s(-1) for d11-cyclohexoxy (228-267 K) independent of pressure in the range 50-90 Torr. The room-temperature rate constant for the reaction of trans-4-methylcyclohexoxy radical with O2 (obtained from the Arrhenius fit) is consistent with the commonly recommended value, but the observed activation energy is approximately 3 times larger than the recommended value of 0.4 kcal/mol and half the value previously found for the reaction of normal cyclohexoxy radical with O2.     

Fine structure of the (S(1)<--S(0)) band origins of phthalocyanine molecules in helium droplets

The laser-induced fluorescence (LIF) excitation spectra of free base phthalocyanine (Pc), Mg-Pc, and Zn-Pc molecules in superfluid helium droplets at T=0.38 K have been studied. The spectra reveal the rich vibronic structure of the S(1)<--S(0) electronic transitions. The band origins of the transitions consist of zero phonon lines accompanied by phonon wings, which originate from simultaneous electronic excitation of the molecule and excitation of the collective modes of the helium surrounding it. The phonon wings have discrete structures suggesting localization of some helium atoms in the neighborhood of the molecules. Zero phonon lines of Mg-Pc and Zn-Pc molecules are split into three components, which are separated by 0.2-0.4 cm(-1). Possible mechanism of splitting involves static or dynamic Jahn-Teller interaction of metal-phthalocyanine molecules in the twofold degenerate S(1)((1)E(u)) state with the helium shell.     

Crystallographic distinction between "contact" and "separated" ion pairs: structural effects on electronic/ESR spectra of alkali-metal nitrobenzenides

The classic nitrobenzene anion-radical (NB(-*) or nitrobenzenide) is isolated for the first time as pure crystalline alkali-metal salts. The deliberate use of the supporting ligands 18-crown-6 and [2.2.2]cryptand allows the selective formation of contact ion pairs designated as (crown)M(+)NB(-*), where M(+) = K(+), Rb(+), and Cs(+), as well as the separated ion pair K(cryptand)(+)NB(-*)-both series of which are structurally characterized by precise low-temperature X-ray crystallography, ESR analysis, and UV-vis spectroscopy. The unusually delocalized structure of NB(-*) in the separated ion pair follows from the drastically shortened N-C bond and marked quinonoidal distortion of the benzenoid ring to signify complete (95%) electronic conjugation with the nitro substituent. On the other hand, the formation of contact ion pairs results in the substantial decrease of electronic conjugation in inverse order with cation size (K(+) > Rb(+)) owing to increased localization of negative charge from partial (NO(2)) bonding to the alkali-metal cation. Such a loss in electronic conjugation (or reverse charge transfer) may be counterintuitive, but it is in agreement with the distribution of odd-electron spin electron density from the ESR data and with the hypsochromic shift of the characteristic absorption band in the electronic spectra. Most importantly, this crystallographic study underscores the importance of ion-pair structure on the intrinsic property (and thus reactivity) of the component ions-as focused here on the nitrobenzenide anion.     

Homo- and heteronuclear alkali metal trimers formed on helium nanodroplets. Part I. Vibronic spectra simulations based on ab initio calculations

We compare the 3(4)A(1) and 4(4)B(2) states of homonuclear and heteronuclear alkali trimers formed of potassium and rubidium. The Multireference Rayleigh Schr?dinger Perturbation Theory of second order is applied to obtain the corresponding adiabatic potential energy surfaces. In the case of homonuclear trimers these pairs of states correspond to the two branches of the E?e Jahn-Teller distorted 2(4)E' state. For heteronuclear trimers, the vibrational modes Q(x) and Q(y) are no longer degenerate, but the two electronic states still show a conical intersection at obtuse (KRb(2)) or acute (K(2)Rb) isosceles geometries. Spectroscopic consequences of this situation are discussed, vibronic spectra are predicted and compared to laser-induced fluorescence spectra obtained in helium droplet isolation spectroscopy experiments of our group.     

Oxalate-bridged complexes of dimolybdenum and ditungsten supported by pivalate ligands: ((t)BuCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)C(t)Bu)(3). Correlation of the solid-state, molecular, and electronic structures with Raman, resonance Raman, and electronic spectral data

The compounds ((t)BuCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)C(t)Bu)(3) (M(4)OXA), where M = Mo or W, are shown by analysis of powder X-ray diffraction data to have extended lattice structures wherein oxygen atoms from the oxalate and pivalate ligands of one M(4)OXA molecule are linked to metal atoms of neighboring molecules. Raman, resonance Raman, electronic absorption (2-325 K in 2-MeTHF), and emission spectra are reported, together with corresponding spectra of the mu-O(2)(13)C(13)CO(2) isotopomers. To aid in the assignment, the Raman spectra of K(2)C(2)O(4).H(2)O and K(2)(13)C(2)O(4).H(2)O have also been recorded. The visible region of the electronic spectra is dominated by intense, fully allowed MLCT transitions, M(2) delta to oxalate pi*, which show pronounced thermochromism and extensive vibronic progressions associated with the oxalate ligand at low temperatures. With excitation into these charge-transfer bands, strong resonance enhancement is seen for Raman bands assigned to the oxalate nu(1)(a(g)) and, to a lesser extent, nu(2)(a(g)) modes. Electronic structure calculations for the model compounds (HCO(2))(3)M(2)(mu-O(2)CCO(2))M(2)(O(2)CH)(3), employing density functional theory (gradient corrected and time-dependent) with the Gaussian 98 and ADF 2000 packages, predict the planar oxalate D(2h) configuration to be favored, which maximizes M(2) delta to oxalate pi* back-bonding, and indicate low barriers (<8 kcal mol(-1)) to rotation about the oxalate C-C bonds.     

Laser induced fluorescence spectroscopy of the C3N radical

Electronic spectra of the C3N radical have been observed for the first time in the near ultraviolet wavelength region by laser induced fluorescence (LIF) spectroscopy. Seventeen vibronic bands of the B 2Pii-X 2Sigma+ electronic transition system of C3N were identified in LIF spectra of products in a discharge of HC3N. The origin of the B 2Pii state was determined to be 27,929.985(1) cm(-1) from rovibrational analyses. It was found that observations of two types of 2Sigma vibronic levels, which have 2Sigma+ and 2Sigma+/- symmetries originated from excitations of the nu4 trans-bending mode (omega4=369.1(20) cm(-1)) with a large Renner-Teller (RT) interaction (epsilon4=-0.1549(50)), and the nu5 cis-bending mode (omega5=163.24(84) cm(-1)) with a small Renner-Teller interaction (epsilon5=-0.0503(68)), respectively. Vibronic levels, with excitations of the C-C stretching (omega3=869.7 cm(-1)) mode, were also identified. The spin-orbit interaction constant was determined to be Aso=-36.7(50) cm(-1) from the RT analysis. In dispersed fluorescence spectra from B 2Pii, vibrational structures of the low-lying electronically excited A 2Pii state were clearly observed with a strong progression due to the nu3' mode, together with those of the X 2Sigma+ state with weak intensities. The origin of A 2Pii, T0=1844(3) cm(-1), and the vibrational frequencies, omega3'=883(3) cm(-1) and omega5'=121(3) cm(-1) for A 2Pii, and omega3"=1054(3) cm(-1), omega4"=405(3) cm(-1), and omega5"=131(3) cm(-1) for X 2Sigma+, were determined. Time profiles of fluorescence from B 2Pii have short (50-200 ns) and long (>1 micros) decay components with quantum beats, indicating that there is a competition between radiative decay and the nonradiative internal conversion to vibrationally highly excited A 2Pii and X 2Sigma+.     

Observation of the 3(3)Sigma(+)-X1Sigma+ transition of KRb by resonance enhanced two-photon ionization in a pulsed molecular beam: Hyperfine structures of 39K85Rb and 39K87Rb isotopomers

The 3(3)Sigma(+)-X1Sigma+ transition of KRb is observed by resonance enhanced two-photon ionization in a pulsed molecular beam. Hyperfine splittings of 39K85Rb and 39K87Rb isotopomers are observed. From the magnitude of hyperfine splittings, we found that the main hyperfine structure was dominated by the Fermi contact interaction between the Rb nuclear spin and the unpaired electron spin. The Fermi contact interaction constants were determined to be 291 MHz for 39K85Rb and 665 MHz for 39K87Rb. In the KRb 3(3)Sigma+ state the electron spin couples more strongly with the Rb nuclear spin than with other angular momenta, and the energy level structure is well described by the hyperfine angular momentum coupling scheme of the b(betaS) case. The molecular constants and the Rydberg-Klein-Rees potential energy curve of the 3(3)Sigma+ state were determined.     

Adduct formation between alkali metal ions and anionic LV(V)O(2)(-) (L(2-) = tridentate ONS ligands) species: syntheses, structural investigation, and photochemical studies

Syntheses of alkali metal adducts [LVO(2)M(H(2)O)(n)] (1-7) (M = Na(+), K(+), Rb(+), and Cs(+); L = L(1)(-)L(3)) of anionic cis-dioxovanadium(V) species (LVO(2)(-)) of tridentate dithiocarbazate-based Schiff base ligands H(2)L (S-methyl-3-((5-(R-2-hydroxyphenyl))methyl)dithiocarbazate, R = H, L = L(1); R = NO(2), L = L(2); R = Br, L = L(3)) have been reported. The LVO(2)(-) moieties here behave like an analogue of carboxylate group and have displayed interesting variations in their binding pattern with the change in size of the alkali metal ions as revealed in the solid state from the X-ray crystallographic analysis of 1, 3, 6, and 7. The compounds have extended chain structures, forming ion channels, and are stabilized by strong Coulombic and hydrogen-bonded interactions. The number of coordinated water molecules in [LVO(2)M(H(2)O)(n)] decreases as the charge density on the alkali metal ion decreases (n = 3.5 for Na(+) and 1 for K(+) and Rb(+), while, for Cs(+), no coordinated water molecule is present). In solution, compounds 1-7 are stable in water and methanol, while in aprotic solvents of higher donor strengths, viz. CH(3)CN, DMF and DMSO, they undergo photoinduced reduction when exposed to visible light, yielding green solutions from their initial yellow color. The putative product is a mixed-oxidation (mu-oxo)divanadium(IV/V) species as revealed from EPR, electronic spectroscopy, dynamic (1)H NMR, and redox studies.     

Synthesis, crystal structures, and luminescence of organic-lanthanide complexes with nicotinate and isonicotinate ligands

Six lanthanide coordination compounds with two isomeric carboxylic acids, nicotinic acid (HL(1)) and isonicotinic acid (HL(2)), [(L(1))3Ln(H2O)2]2 (Ln = Eu, 1; Gd, 2; Tb, 3) and [( L(2))2Ln(H2O)4][NO3] (Ln = Eu, 4; Gd, 5; Tb, 6), have been synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1-3 are dimeric whereas 4-6 are polymeric, all with 8-coordination of Ln(3+). The distinction between these lanthanide complexes is readily accomplished from the 10 K high resolution electronic emission spectra. Spectral interpretation is given for the Eu(3+) complexes 1, 4, whereas the spectra of 3 and 6 are more complex. The relationships between spectroscopic and crystallographic site symmetries are discussed. The calculated second rank crystal field strengths of Eu(3+) in 1 and 4 are intermediate in magnitude.     

Structure of selected basic zinc/copper (II) phosphate minerals based upon near-infrared spectroscopy--implications for hydrogen bonding

The NIR spectra of reichenbachite, scholzite and parascholzite have been studied at 298 K. The spectra of the minerals are different, in line with composition and crystal structural variations. Cation substitution effects are significant in their electronic spectra and three distinctly different electronic transition bands are observed in the near-infrared spectra at high wavenumbers in the 12,000-7600 cm(-1) spectral region. Reichenbachite electronic spectrum is characterised by Cu(II) transition bands at 9755 and 7520 cm(-1). A broad spectral feature observed for ferrous ion in the 12,000-9000 cm(-1) region both in scholzite and parascholzite. Some what similarities in the vibrational spectra of the three phosphate minerals are observed particularly in the OH stretching region. The observation of strong band at 5090 cm(-1) indicates strong hydrogen bonding in the structure of the dimorphs, scholzite and parascholzite. The three phosphates exhibit overlapping bands in the 4800-4000 cm(-1) region resulting from the combinations of vibrational modes of (PO(4))(3-) units.     

Formation of ultracold metastable RbCs molecules by short-range photoassociation

Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a?(3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].     

UV and IR spectroscopic studies of cold alkali metal ion-crown ether complexes in the gas phase

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li(+), Na(+), K(+), Rb(+), and Cs(+)) in a cold, 22-pole ion trap. All the complexes show a number of vibronically resolved UV bands in the 36,000-38,000 cm(-1) region. The Li(+) and Na(+) complexes each exhibit two stable conformations in the cold ion trap (as verified by IR-UV double resonance), whereas the K(+), Rb(+), and Cs(+) complexes exist in a single conformation. We analyze the structure of the conformers with the aid of density functional theory (DFT) calculations. In the Li(+) and Na(+) complexes, DB18C6 distorts the ether ring to fit the cavity size to the small diameter of Li(+) and Na(+). In the complexes with K(+), Rb(+), and Cs(+), DB18C6 adopts a boat-type (C(2v)) open conformation. The K(+) ion is captured in the cavity of the open conformer thanks to the optimum matching between the cavity size and the ion diameter. The Rb(+) and Cs(+) ions sit on top of the ether ring because they are too large to enter the cavity of the open conformer. According to time-dependent DFT calculations, complexes that are highly distorted to hold metal ions open the ether ring upon S(1)-S(0) excitation, and this is confirmed by extensive low-frequency progressions in the UVPD spectra.