Predissociation Dynamics of B State of Methyl Iodide with Femtosecond Pump-probe Technique

The predissociation dynamics of B Rydberg state of methyl iodide is studied with femtosecond two-color pump-probe time-of-°ight spectra at pump pulse of 400 nm and probe pulse of 800 nm. The dominant product channels are the CH3I+ and CH3+ formation. The time-dependent signals for CH3I+ and CH3+ ions are obtained. Both of the signal curves can be ˉtted by biexponential decays with time constants of ?1 and ?2, ?1 is assigned to the lifetimes of high Rydberg states, which can be accessed by absorbing three 400 nm pump pulses and ?2 re°ects the dynamics of B Rydberg state, which is accessed with two pump pulses. The lifetime of B Rydberg state is determined to be about 1.57 ps, which is incredibly consistent with the previous studies. The results were interpreted as a multiphoton dissociative ionization processes.     

Femtosecond multiphoton ionization of pyrrole

The photoionization dynamics of pyrrole are investigated by using a photoelectron imaging method and a tunable femtosecond laser. Two-photon nonresonant ionization experiments in the wavelength range from 261 to 298 nm indicate that the cation and neutral ground states have similar structures. The main vibrational excitation in the cation ground state is the v(8) mode. Two-photon absorption at 406 nm projects neutral pyrrole into a mixed state comprising the 1B(2) valence and 3p Rydberg states. Ionization from this mixed state mainly results in the overtone excitation of vibrational mode v(8) and v(9) of the cation state. In the wavelength range from 336 to 364 nm, a mixed state comprising the 3d/4s Rydberg and the 4A(1) valence states are populated by the absorption of two photons through vibronic coupling. The partition ratio among these states varies with the excitation wavelength, resulting in dramatic changes in both kinetic energy distributions and angular distributions. As the laser wavelength becomes shorter, from 336 to 314 nm, higher excited states, 3B(2), 5A(1), 6A(1), 7B(1) and 4B(2), can be populated. Photoelectron angular distributions provide supplementary verification of assignments. Our experiments indicate that femtosecond multiphoton ionization and photoelectron imaging methods are powerful tools for investigating short-lived intermediated excited states, which cannot be detected in nanosecond experiments.     

Two-color multiphoton ionization spectra of jet-cooled p-difluorobenzene - s and d Rydberg states

Two-color multiphoton ionization spectra of jet-cooled p-difluorobenzene due to the transitions from S₁ state to highly excited Rydberg states have been observed. At least six different Rydberg series of s and d characters were found. The results indicate a reduction of molecular symmetry in the Rydberg state to C_2h. The Rydberg states belonging to different series exhibit different ionization behavior.     

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.     

Even-parity auto-ionizing states of iridium I observed by resonance laser ionization spectroscopy

Auto-ionizing states of neutral iridium were observed in the continuum structure near the first ionization limit using one-color and two-color two-step resonance laser ionization spectroscopy. The total angular momentum of 20 even-parity auto-ionizing states could be determined from a combined analysis of the two-color spectra obtained with ionization schemes using intermediate states with different total angular momentum. Double-resonant ionization schemes were evaluated by fluence-dependence measurements, and photo-ionization cross-sections for resonant ionization transitions were determined. We could also identify several high-lying members of ns, np and nd Rydberg series converging to the first ionization limit of the atom.     

Rydberg spectra of trans-1,2-dibromoethylene

(2+1) resonance-enhanced multiphoton ionization spectra of jet-cooled trans-1,2-dibromoethylene are reported for the first time. The two-photon spectral region between 149.7 and 141.2 nm was examined. A 4p(z)<--pi Rydberg transition between 66,800 and 68,000 cm(-1) with A(g) excited state symmetry was analyzed, as well as two 4f<--pi Rydberg transitions with B(g) excited state symmetry and one 4f<--pi Rydberg transition with A(g) excited state symmetry between 68,000 and 70,800 cm(-1). All Rydberg transitions observed in this work belong to series that converge to the first ionization potential of the molecule. The short vibrational progressions observed involve two totally symmetric in-plane normal modes: C=C-H bending (nu(3)) and C=C-Br bending (nu(5)) with average excited state frequencies of 829 and 226 cm(-1), respectively.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

Pulsed-field ionization spectroscopy of high Rydberg states (n=50-200) of bis(eta6-benzene)chromium

The ionization behavior of the high Rydberg states of bis(eta(6)-benzene)chromium in the presence of ac and/or dc fields has been explored. The application of an ac scrambling field at the time of laser excitation lengthens the lifetime of the Rydberg state by almost two orders of magnitude. The lifetime enhancement by the scrambling field is much more effective for n<100 than it is for n>100 Rydberg states. The pulsed-field ionization of Rydberg states of n<100 shows the typical diabatic ionization behavior for low n. The two distinct ionization behaviors observed for the relatively low (n=50-100) and high (n=100-200) Rydberg states suggest that the former originate from the optically accessed nf Rydberg series, whereas the latter are due to np Rydberg series. Based on the understanding of the ionization behavior of bis(eta(6)-benzene)chromium, the accurate ionization potential is deduced to give IP=5.4665+/-0.0003 eV. Optimization of the various electric field conditions greatly enhances the spectral sensitivity of the mass-analyzed threshold ionization (MATI) spectroscopy. The high-resolution MATI spectrum of the title molecule obtained here provides precise cationic vibrational frequencies for many skeletal and benzene ring modes. A number of vibrational modes are newly identified, and the ambiguity regarding to some mode assignments is now clearly resolved through the Frank-Condon analysis based on ab initio calculations.     

Multiphoton dissociation dynamics of BrCl and the BrCl+ cation

Ion imaging methods have enabled identification of three mechanisms by which (79)Br(+) and (35)Cl(+) fragment ions are formed following one-color multiphoton excitation of BrCl molecules in the wavelength range 324.6 > lambda > 311.7 nm. Two-photon excitation within this range populates selected vibrational levels (v'= 0-5) of the [X (2)Pi(1/2)]5ssigma Rydberg state. Absorption of a third photon results in branching between (i) photoionization (i.e. removal of the Rydberg electron-a traditional 2 + 1 REMPI process) and (ii)pi*<--pi excitation within the core, resulting in formation of one or more super-excited states with Omega= 1 and configuration [A (2)Pi(1/2)]5ssigma. The fate of the latter states involves a further branching. They can autoionize (yielding BrCl(+)(X (2)Pi) ions in a wider range of v(+) states than formed by direct 2 + 1 REMPI). Further, one-photon absorption by the parent ions resulting from direct ionization or autoionization leads to formation of Br(+) and (energy permitting) Cl(+) fragment ions. Alternatively, the super-excited molecules can fragment to neutral atoms, one of which is in a Rydberg state. Complementary ab initio calculations lead to the conclusion that the observed [Cl**[(3)P(J)]4s + Br/Br*] products result from direct dissociation of the photo-prepared super-excited states, whereas [Br**[(3)P(J)]5p + Cl/Cl*] product formation involves interaction between the [A (2)Pi(1/2)]5ssigma and [X (2)Pi(1/2)]5psigma Rydberg potentials at extended Br-Cl bond lengths. Absorption of one further photon by the resulting Br** and Cl** Rydberg atoms leads to their ionization, and thus their appearance in the Br(+) and Cl(+) fragment ion images.     

Two-step resonant photoionization spectroscopy of Co I and Ni I: continuum structure near the first ionization limit

We explored the continuum structure of Co I and Ni I near the first ionization limit using one-color and two-color two-step photoionization spectroscopy. The ionization spectra of both elements show a large number of autoionizing features. The use of different excitation schemes involving intermediate states with different angular momentum allowed the assignment of the angular momentum for part of the detected autoionizing states. Several of these states could be identified as members of different Rydberg series.     

Multireference configuration interaction studies on higher valence and Rydberg states of OClO, ionization potentials, and electron detachment energies

MRCI results are reported for the vertical excitation energies (VEE) and oscillator strengths f of doublet states of OClO up to 11 eV, including 3b(1) → 4s, 4p, 3d, 5s, 5p, 4d, and most 1a(2), 8a(1), 5b(2) → 4s and 4p Rydberg states. The lowest Rydberg states 3b(1) → 4s and 3b(1) → 4p(x) have mixed valence-Rydberg character. The observed spectral bands were reassigned to include valence states which have generally higher oscillator strengths. The well-known valence state 1(2)A(2) has a VEE of 3.63 eV, and a relatively high f of 0.042. Overall, the calculated oscillator strengths are in good agreement with measured values. The lowest quartet state, 1(4)B(2), lies at 6.95 eV. Quartet Rydberg states start with 1a(2) → 4s at 9.28 eV. According to calculated vertical ionization potentials (VIP) of OClO, the second VIP at 12.59 eV is reassigned from 1(3)B(1) to 1(3)B(2) (ionization from 1a(2), rather than 8a(1)), and the third VIP at 12.63 eV from 1(1)B(1) to 1(3)B(1) (ionization from 8a(1)). Vertical electron detachment energies of OClO(-) have been calculated up to 8.9 eV. There is good agreement with experimental values.     

Energy flow and fragmentation dynamics of n,n-dimethylisopropylamine

The energy flow and fragmentation dynamics of N,N-dimethylisopropylamine (DMIPA) upon excitation to the 3p Rydberg states has been investigated with use of time-resolved photoelectron and mass spectrometry. The 3p states are short-lived, with a lifetime of 701 +/- 45 fs. From the time dependence of the photoelectron spectra, we infer that the primary reaction channel leads to the 3s level, which itself decays to the ground state with a decay time of 87.9 +/- 10.2 ps. The mass spectrum reveals fragmentation with cleavage at the alpha C-C bond, indicating that the energy deposited in vibrations during the internal conversion from 3p to 3s exceeds the bond energy. A thorough examination of the binding energies and temporal dynamics of the Rydberg states, as well as a comparison to the related fragmentation of N,N-dimethyl-2-butanamine (DM2BA), suggests that the fragments are formed on the ion surfaces, i.e., after ionization and on a time scale much slower than the fluorescence decay from 3s to the ground state.     

Excited states of the water molecule: analysis of the valence and Rydberg character

The excited states of the water molecule have been analyzed by using the extended quantum-chemical multistate CASPT2 method, namely, MS-CASPT2, in conjunction with large one-electron basis sets of atomic natural orbital type. The study includes 13 singlet and triplet excited states, both valence and 3s-, 3p-, and 3d-members of the Rydberg series converging to the lowest ionization potential and the 3s- and 3p-Rydberg members converging to the second low-lying state of the cation, 1 (2)A(1). The research has been focused on the analysis of the valence or Rydberg character of the low-lying states. The computation of the 1 (1)B(1) state of water at different geometries indicates that it has a predominant 3s-Rydberg character at the equilibrium geometry of the molecule but it becomes progressively a valence state described mainly by the one-electron 1b(1)-->4a(1) promotion, as expected from a textbook of general chemistry, upon elongation of the O-H bonds. The described valence-Rydberg mixing is established to be originated by a molecular orbital (MO) Rydbergization process, as suggested earlier by R. S. Mulliken [Acc. Chem. Res. 9, 7 (1976)]. The same phenomenon occurs also for the 1 (1)A(2) state whereas a more complex behavior has been determined for the 2 (1)A(1) state, where both MO Rydbergization and configurational mixing take place. Similar conclusions have been obtained for the triplet states of the molecule.     

Conrotatory photochemical ring opening of alkylcyclobutenes in solution. A test of the hot ground-state mechanism.

Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest pi,R(3s) excited singlet states of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry. The compounds studied in this work-1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-5), hexamethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.0(2,7)]dodec-1(2)-ene (cis- and trans-9)-were selected so as to span a broad range in molecular weight and as broad a range as possible in Arrhenius parameters for thermal (ground-state) ring opening. RRKM calculations have been carried out to provide estimates of the rate constants for ground-state ring opening of each of the compounds over a range of thermal energies from 20 000 to 49 000 cm(-1). These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground-state mechanism, assuming a value of k(deact) = 10(11) s(-1) for the rate constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest Rydberg state occurs with close to unit efficiency, and that ergodic behavior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the Rydberg-derived ring opening of alkylcyclobutenes is a true excited-state process and rules out the hot ground-state mechanism for the reaction.     

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.     

Two-color excitation of NO in a supersonic free jet. Autoionization of high rydberg states

High Rydberg states of NO above the ionization limit have been measured for the isolated molecule in a supersonic free jet by two-color multiphoton ionization. Three Rydberg series (ns, np and nf) were identified, which appeared by rotational and the vibrational autoionization. The rotational structures of the 13s(υ = 1), 13p(υ = 1) and 12f(υ = 1) states have been analyzed in detail. The fluorescence dip spectra for the intermediate A²Σ⁺(3sσ) state have been measured simultaneously and the cross sections of the one-photon absorption to the high Rydberg states from the A²Σ⁺(υ = 1) state have been determined.     

Two-photon dissociation of the NO dimer in the region 7.1-8.2 eV: excited states and photodissociation pathways

A study of excited states of the NO dimer is carried out at 7.1-8.2 eV excitation energies. Photoexcitation is achieved by two-photon absorption at 300-345 nm followed by (NO)(2) dissociation and detection of electronically excited products, mostly in n=3 Rydberg states of NO. Photoelectron imaging is used as a tool to identify product electronic states by using non-state-selective ionization. Photofragment ion imaging is used to characterize product translational energy and angular distributions. Evidence for production of NO(A (2)Sigma(+)), NO(C (2)Pi), and NO(D (2)Sigma(+)) Rydberg states of NO, as well as the valence NO(B (2)Pi) state, is obtained. On the basis of product translational energy and angular distributions, it is possible to characterize the excited state(s) accessed in this region, which must possess a significant Rydberg character.     

One-color two-photon mass-analyzed threshold ionization spectroscopy of ethyl bromide through a dissociative intermediate state

Mass-analyzed threshold ionization (MATI) spectra of ethyl bromide were obtained using one-color two-photon ionization through a dissociative intermediate state. Accurate values for the adiabatic ionization energy have been obtained, 83099+/-5 and 85454+/-5 cm(-1) for the X1 2E and X2 2E states of the ethyl bromide cation, respectively, giving a splitting of 2355+/-10 cm(-1). Compared with conventional photoelectron data, the two-photon MATI spectrum exhibited a more extensive vibrational structure with a higher resolution, mainly containing the modes involving the dissociation coordinate. The observed modes were analyzed and discussed in terms of wave packet evolving on the potential-energy surface of the dissociative state.     

Neutral photodissociation of superexcited states of molecular iodine

The formation of high-n Rydberg atoms from the neutral dissociation of superexcited states of I(2) formed by resonant two-photon excitation of molecular iodine using an ArF laser has been investigated. The high-n Rydberg atoms I* are formed by predissociation of the optically excited molecular Rydberg states I*(2)[R(B (2)Sigma(g) (+))] converging on the I(2) (+)(B (2)Sigma(g) (+)) state of the ion. Measurement of the kinetic energy release of the Rydberg I* fragments allowed the identification of the asymptotic channels as I*[R((3)P(J))]+I((2)P(32)), where the I*[R((3)P(J))] are Rydberg atoms converging on the I(+)((3)P(J)) states of the ion with J=2, 1, and 0. In the case of the I*[R((3)P(2))] fragments, the average Rydberg lifetime is observed to be 325+/-25 micros. Based on experiments on the variation of the Rydberg atom signal with the field ionizing strength, the distribution of Rydberg levels peaks at about 25-50 cm(-1) below the ionization limit.     

Vacuum ultraviolet excitation spectroscopy of the autoionizing Rydberg states of atomic sulfur in the 73 350-84 950 cm(-1) frequency range

The photoionization efficiency (PIE) spectra of metastable sulfur (S) atoms in the 1 D and 1 S states have been recorded in the 73 350-84 950 cm(-1) frequency range by using a velocity-mapped ion imaging apparatus that uses a tunable vacuum ultraviolet laser as the ionization source. The S(1 D) and S(1 S) atoms are produced by the 193 nm photodissociation of CS2. The observed PIE spectra of S(1 D) and S(1 S) shows 35 autoionizing resonances with little or no contribution from direct photoionization into the S+(4S 3/2)+e(-) ionization continuum. Velocity-mapped ion images of the S+ at the individual autoionizing Rydberg resonances are used to distinguish whether the lower state of the resonance originates from the 1 D, 1 S, or 3P states. The analysis and assignment of the Rydberg peaks revealed 22 new Rydberg states that were not previously known. The autoionization lifetimes tau of the Rydberg states are derived from the linewidths by fitting the lines with the Fano formula. Deviations from the scaling law of tau(n*) proportional to, n*3, where n* is the effective quantum number of the Rydberg state, are observed. This observation is ascribed to perturbations by nearby triplet Rydberg states, which shorten the autoionization lifetimes of the singlet Rydberg levels.