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The excited state photodynamics of isolated benzene have been studied in the nanosecond range by two-step photoionization through various vibrations of the lowest singlet state, with imaging photoelectron spectroscopy detection. Photoelectron spectra are measured as a function of pump–probe delay time, and their time evolution is successfully compared to a biexponential decay function without regard to a particular kinetic model. The only reasonable kinetic model with only two exponentials is the one that involves an intersystem crossing from S1 to T1, although that model has previously been called into question by high-resolution studies that failed to find any singlet–triplet perturbations in Zeeman studies of the S1 spectrum. That contradiction remains unresolved. [ABSTRACT FROM AUTHOR]

Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm-1 of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics. [ABSTRACT FROM AUTHOR]

A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B 2B2-X 2B1 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained. [ABSTRACT FROM AUTHOR]

Photoinduced Rydberg ionization (PIRI) spectra of the second excited electronic state of benzonitrile cation were recorded via the origin and 6a1 and 6b1 vibrational levels of the cation ground electronic state. This B←X transition was verified to be a forbidden 2B2←2B1 transition with an origin at 17 225 cm-1 above the ground ionic state. By the use of vibronic coupling calculations, as well as symmetry analysis and comparison of the PIRI spectra via different ground vibrational levels, a nearly complete assignment of the vibrational structure was made, and the vibrational frequencies of the B 2B2 state of benzonitrile cation were obtained based on the assignments. Comparisons of the experimental spectra with simulations from the vibronic structure calculations are also used to validate the theoretical procedures used in the simulations. [ABSTRACT FROM AUTHOR]

New mass analyzed threshold ionization (MATI) spectra of the molecules C6H6+ and C6D6+ have been collected using tunable vacuum ultraviolet (VUV) single photon excitation from the neutral ground state and also using two-photon excitation through the 61 vibration of the 1B2u S1 state. Emphasis was placed on obtaining accurate relative intensities of the vibrational lines in order to use this information in the vibronic analysis. The MATI spectra collected from VUV (S0 originating state), triplet (T1), and resonant two photon (S1) excitation schemes were compared with Jahn–Teller calculations employing the classical model of Longuet–Higgins and Moffitt to obtain the Jahn–Teller coupling parameters of 3 of the 4 linearly active modes (e2g modes 6–9 in Wilson’s notation). Franck–Condon factors, including the effects of geometry changes, were calculated from the vibronic wave functions and used to identify the lines in the various spectra. It is found that most of the lines with substantial intensity can be understood using only the modes 1, 6, 8, and 9. Weaker peaks are due to various non-e2g modes, but these do not derive intensity through Jahn–Teller coupling. When the effects of geometry change were included, simulations of the spectra from the calculated vibrational energies and intensities were close to the experimental spectra. This verifies the applicability of the model to the understanding of the vibrational structure of this type of molecule, but some variations indicate directions for further improvement of the model. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The single-photon pulsed field ionization photoelectron (PFI-PE) spectrum of trans-2-butene (trans-CH[sub 3]CH=CHCH[sub 3]) in the energy range of 73 500–75 850 cm-1 has been measured using vacuum ultraviolet laser sources. The semi-empirical simulation of fine structures resolved in the original PFI-PE band yields a value of 73 624.7±2.0 cm[sup -1] for the ionization energy (IE) of trans-2-butene. The vibrational bands for trans-CH[sub 3]CH=CHCH[sub 3][sup +] resolved in the PFI-PE spectrum are assigned based on ab initio calculations of the vibrational frequencies and Franck-Condon factors (FCFs) for ionization transitions. This assignment has provided reliable vibrational frequencies (ν[sub 1][sup +]=104 cm[sup -1], ν[sub 2][sup +]=127 cm[sup -1], ν[sub 3][sup +]=131 cm[sup -1], ν[sub 5][sup +]=484 cm[sup -1], ν[sub 8][sup +]=798 cm[sup -1], ν[sub 13][sup +]=1164 cm[sup -1], ν[sub 14][sup +]=1264 cm[sup -1], ν[sub 16][sup +]=1307 cm[sup -1], ν[sub 20][sup +]=1407 cm[sup -1], and ν[sub 22][sup +]=1567 cm[sup -1]) for trans-CH[sub 3]CH=CHCH[sub 3][sup +]. The PFI-PE spectrum is compared to the recently reported PFI-photoion (PFI-PI) spectrum for trans-2-butene. The major difference observed between the PFI-PE and PFI-PI spectra is that the intensities for excited vibrational bands were significantly suppressed or indiscernible in the PFI-PI spectrum, suggesting that the lifetimes for high-n Rydberg states associated with these excited vibrational bands were greatly reduced under the conditions used in the PFI-PI study. The experimental conditions used in the PFI-PI study also led to an IE value of about 20 cm-1 lower than that obtained in the PFI-PE measurement. We have also reassigned the vibrational bands resolved in the PFI-PE spectrum for cis-2-butene based on the FCF calculation and a more reliable set of theoretical vibrational frequencies. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Jahn-Teller (JT) coupling parameters can be simply derived from features of the potential energy surfaces of the JT active vibrational modes of a molecule subject to that effect. Potential energy curves representing cuts of D[SUB2h] symmetry have been calculated using density functional theory for each of the four active e[SUB2g] modes of benzene cation in each of the lowest three degenerate electronic states. From these curves and the vibrational frequencies, the JT coupling parameters can be found (vibrational numbering follows Wilson's notation, coupling parameters are in Moffit's notation). For the C[SUB6]H[SUP+,SUB6]+&Xtilde;[SUP2]E[SUB1g] state the largest JT coupling parameter is provided by mode 6 (D = 0.48), followed closely by mode 8 (0.30) and then mode 9 (0.10). Mode 7 provides almost no stabilization. For the [SUP2]E[SUB2g] state, calculated coupling parameters are very large, particularly for modes 8 (1.36) and 6 (0.93). Modes 7 (0.10) and 9 (0.07) are smaller but finite. For the &Dtilde;[SUP2]E[SUBlu] state, mode 6 has an impressive linear coupling parameter of 4.12, able to support several vibrations below the JT cusp. Indications of a substantial quadratic stabilization for that mode result in the possibility of a completely static distortion in that state. Similar, but not identical, results are obtained for C[SUB6]D[SUP+,SUB6] . [ABSTRACT FROM AUTHOR]

The vibrational structure seen in photoinduced Rydberg ionization spectra of the &Btilde;[SUP2]E[SUB2g] state of benzene cation (C[SUB6]H[SUP+,SUB6] and C[SUB6]D[SUP+,SUB6]) has been analyzed by fitting the vibrational patterns to energy levels derived from multimode Jahn-Teller calculations. Most of the structure can be ascribed to various combinations of modes 6 and 16, with minor contributions from 4, 17, and 18 (using Wilson's numbering convention). In qualitative agreement with parameters derived from electronic structure calculations, the linear coupling parameter for mode 6 is very large (D = 1.39 in C[SUB6]H[SUP+,SUB6] and 1.28 in + C[SUB6]D[SUB6] ). This raises questions about certain angular momentum selection rules in the classical Jahn-Teller model. [ABSTRACT FROM AUTHOR]

The origin of the cation ground electronic state was used to record the photoinduced Rydberg ionization spectrum of third excited electronic state of phenylacetylene cation. It was found that the vibrational structure of C state of phenylacetylene cation is well resolved, although the line widths are not quite as narrow as other substituted benzenes.

Describes the interaction between molecules of ethyl radical with small torsional barrier. Range of action between internal rotation and other normal modes of the molecule; Use of electronic structure calculations to estimate both the electronic and vibrational contributions to the barrier.

Reports the infrared spectrum measurements of ethyl radical carbon hydrogen[sub 2] out-of-plane rocking vibrational fundamental by transient diode laser absorption spectroscopy. Data providing information on the structure of and the barrier of internal rotation; Examination of how quantities change on vibrational excitation.

The formation of neutral molybdenum, carbide, nitride and sulfide clusters and nanocrystallites using a laser ablation plasma reactor is investigated. Mass distributions, stability, and reactivity of neutral and charged clusters of a wide range of transition-metal compounds consisting of a few atoms to very large particles containing more than 100 metal atoms is explored.

The observation and analysis of the high resolution spectrum of the CH2 rocking fundamental of the ethyl radical, C2H5, at wavelengths close to 18.9 μm is reported. The band origin is found to be at 528.1 cm-1. The spectrum shows evidence for a very low barrier to internal rotation, or torsion, in this species. A simple model Hamiltonian, based on an assumed structure with G12 symmetry, qualitatively reproduces the observations and implies a torsional barrier of approximately 20 cm-1 in both the zero point and excited vibrational states. The Hamiltonian cannot reproduce the observations to the level of the experimental accuracy and the importance of several neglected terms is tested and discussed. The observed torsional splittings imply that, within the confines of the model, the a-inertial and internal rotation axes in the molecule are coincident to within 1°. In addition to their intrinsic interest, the spectra will be useful for future state resolved studies of the kinetics of chemical reactions involving the radical. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photofragmentation of the chlorofluorocarbons, CF3Cl, CF2Cl2, and CFCl3, was investigated at 187, 125, and 118 nm using VUV harmonic generation techniques and (2+1) resonantly enhanced multiphoton ionization detection of ground Cl(2P3/20) and excited Cl*(2P1/20) state fragment atoms. Product translational energy and angular distributions were derived from Cl+ arrival time distributions obtained by time-of-flight mass spectrometry. Photolysis of CF3Cl at 125 and 118 nm takes place via the 4s(a1) and 4p(e) Rydberg states, respectively, and two primary fragmentation channels are observed. A ‘‘slow’’ channel with a most probable center-of-mass (c.m.) translational energy near zero is assigned to the production CF*3 radicals in the 2A’1, 2A‘2, and 1E’ electronically excited states. The second Cl/Cl* fragmentation channel has a c.m. translational energy distribution peaked at Etr≥1 eV and is tentatively assigned to a sequential dissociation process in which rapid C–Cl single bond rupture is followed by a secondary fragmentation of CF*3 to CF2+F. The time-of-flight (TOF) spectra for CF2Cl2 and CFCl3 following excitation at 125 and 118 nm suggest that concerted three-body fragmentation involving the loss of two Cl/Cl* atoms is the dominant dissociation process. By contrast, photolysis of CF2Cl2 and CFCl3 at 187 nm results in structured Cl+ arrival time distributions which are used to derive translational energy distributions and asymmetry parameters. Simulations of the TOF spectra suggest the presence of three Cl/Cl* fragment channels, with the highest energy channel clearly attributable to single C–Cl bond rupture leading to internally excited molecular fragments. The contribution of sequential and simultaneous two-Cl loss processes to the low translational energy channels is also discussed. [ABSTRACT FROM AUTHOR]

In (3+1) resonance enhanced multiphoton ionization photoelectron spectra (REMPI-PES) of CO2, photoionization competes with dissociation. In addition to direct photoionization, autoionization is possible through accidental resonances embedded in the continuum at the four-photon level. Photoabsorption from these long-lived autoionizing states leads to resonance enhanced above threshold absorption (REATA). REATA produces photoelectron terminations on the C state of CO2+. Previous experiments did not indicate whether the dissociation occurred at the three-photon level or four-photon level. REMPI-PES of CO2 via several Rydberg states have been collected at a number of laser intensities, and it was found that the photoelectron spectra terminating on each individual ionic state do not change over the range of experimentally available laser intensities. This indicates that the dissociation of CO2 occurs at the four-photon level. The long vibrational progressions in the PES indicate that the dominant ionization process is autoionization rather than direct ionization. Relative intensities of the X and C state components of the PES do change with intensity, confirming the C state assignment and its five-photon mechanism. [ABSTRACT FROM AUTHOR]

The MPI spectra of CO2 were recorded in the range 275 to 338 nm in both a conventional MPI cell and a molecular beam apparatus. The polarization ratio of each MPI peak was measured in order to assign the symmetry of excited states. The 3pσu 1Πu and 3pσu 1Πu110 Rydberg states were observed, while the 3pπu 1Σ+u Rydberg state was missing, and a new Rydberg state, 3pπu 1Δu was observed at 333.03 and 333.70 nm. In the nf (n=4,5) Rydberg series, evidence was found of interaction with other nearby Rydberg states. A mass selected pure CO+2 ion signal was measured for the 3pπu 1Δu state, but both CO+ and CO+2 ion signals were detected for the other Rydberg states. Predissociation and fragmentation in MPI processes are discussed. Lower limit lifetimes for the observed Rydberg states of CO2 are given. [ABSTRACT FROM AUTHOR]

Two new (2+1) multiphoton ionization spectra of gerade Rydberg states of molecular iodine are observed in the 310.0–322.5 and the 285.5–295.5 nm ranges. These spectra were obtained in both a conventional MPI cell and in a TOF molecular beam apparatus. A long progression in the 310.0–322.5 nm range is suggested to be 2g(2Π3/2g)ndσg, having a band origin at 62 440 cm-1. Three progressions in the 285.5–295.5 nm range are suggested to be 2g, 1g, 0g(2Π3/2g)(n+1)dσg, having band origins at 68 335, 68 407, and 68 434 cm-1, respectively. The photodissociation behavior of molecular iodine in a MPI process is discussed. [ABSTRACT FROM AUTHOR]

Quantum yields and rates of intersystem crossing and internal conversion are obtained for the lowest excited singlet state of collisionless phenol. It is found that internal conversion is the dominant decay channel, even at the origin of S1, in contrast to previously studied molecules. Also, triplet lifetimes measured as a function of vibrational energy are an order of magnitude shorter than in other isoelectronic aromatics. Hydrogen bonding by water and in a dimer is found to lengthen the singlet lifetime and decrease the quantum yield of internal conversion. This behavior parallels the effects, in solution, of going from a nonpolar to a polar solvent, indicating that substantial solvation effects are caused by a single hydrogen bond. [ABSTRACT FROM AUTHOR]

We report the observation and assignment of the ν2 fundamental vibration in the HOCO and DOCO radicals. The radical was made by the photolysis of acetic acid or acetic acid-d at 193 nm in a flow system. The observed spectra indicate that the excited vibrational level is perturbed in both HOCO and DOCO. In HOCO, the rotational levels in ν2=1 have an irregular dependence on the Ka quantum number, probably caused by anharmonic interactions with combinations of lower frequency in plane vibrations. An a-type Coriolis interaction involving perturbation by a level containing one quantum of the out-of-plane torsional vibration cannot be rigorously excluded however. In DOCO, there are also N-dependent perturbations in the excited state. Only parallel transitions were assigned in the ν2 fundamental of both isotopomers. The K=0 band origin for HOCO is at 1852.567 cm-1 which compares with 1843.7 cm-1 reported previously for the radical trapped in a low-temperature argon matrix. This absorption spectrum will be useful in future studies of chemical reactions involving the radical. [ABSTRACT FROM AUTHOR]

Resonance enhanced multiphoton ionization photoelectron spectra (REMPI-PES) of CO2 were obtained via several Rydberg states. Contrary to expected diagonal Franck–Condon factors, long vibrational progressions were seen in each photoelectron spectrum. The dominant vibration varied with the resonant state. A long progression of two quanta of the asymmetric stretch was observed in the PES through nf resonances, while symmetric stretches were more apparent with the 3p resonances. These results may arise from photoionization occurring during molecular dissociation. [ABSTRACT FROM AUTHOR]

Photoabsorption above the first ionization potential of CO2 was observed at relatively low laser intensity, detected via resonant-enhanced multiphoton ionization-photoelectron spectra through several Rydberg states. This phenomenon can be explained by the presence of accidental resonances with long-lived autoionizing states which make photon absorption within the ionization continuum possible. Laser powers are too low for this to be explained in terms of a ponderomotive potential and conventional above-threshold ionization. This resonance-enhanced above-threshold absorption phenomenon is potentially useful in the study of excited and superexcited states. Photoelectron energies can be assigned to terminations on CO+2 ionic states at both the four- and five-photon levels, allowing measurement of states up to 22 eV. Two unassigned bands may represent terminations on a new state of CO+2, with an ionization potential of 21.4 eV. [ABSTRACT FROM AUTHOR]

A systematic explanation of the pressure-dependent multiphoton ionization spectra of argon is given in terms of the phase matching requirements of third-harmonic generation. Two-color sum-frequency generation in a highly dispersive medium and its effects on normal three-photon excitation process are analyzed, using argon as an example. Finally an anomaly in the resonance line shapes of the two laser MPI spectra of argon is reported and attributed to the displacement of the foci of the two counterpropagating laser beams. [ABSTRACT FROM AUTHOR]

We present a study of the multiphoton ionization spectrum of chlorine in the region of 2 1Πu←X 1Σ+g(1,0–0) three-photon resonances. Spectra at various pressures were obtained with different polarizations and counterpropagating beam experiments were carried out to restore the peaks quenched by third harmonic interference effects. Chlorine atoms, caused by photodissociation, produce prominent lines in the spectrum and are also responsible for third harmonic generation (THG) which in turn excites some spin-forbidden transitions by one-photon absorption. We discuss the change of polarization behavior in the presence of THG and the resultant effect on the symmetry assignments of the excited states. We also discuss third harmonic destructive interference effects in molecular systems and show that total suppression of three-photon transition to one-photon allowed states, common in atoms, will not usually occur in molecules although in general changes in polarization behavior should be observed at higher pressures. This conclusion is demonstrated in the chlorine spectrum. [ABSTRACT FROM AUTHOR]