Volume 97, Issue 9, 01 November 1992
Index of content:

Two‐photon dissociation of HD^{+} with one intermediate resonance in the presence of two different laser fields
View Description Hide DescriptionThe time‐dependent dissociation probabilities, line shapes of dissociation rate, and photofragment kinetic‐energy distribution have been investigated for resonant two‐photon dissociation of HD^{+} from the 1sσ_{ g }(v=0, J=0) state in the presence of two radiation fields of different frequencies ω_{1} and ω_{2}. Simultaneous dissociation in two different electronic states at two different final energies occur through the absorption of either ω_{1}+ω_{2} or 2ω_{2}photons. Only one of the two fields causes a near‐resonant coupling either with (v=6, J=1) through absorption of a photon of frequency ω_{1} or with the (v=14, J=1) state through absorption of a photon of frequency ω_{2}. Dissociation probabilities have been calculated as functions of the resonant and off‐resonant field amplitudes and times. The dissociation probability at fixed times show interesting variation with the field amplitudes. Conditions for time‐dependent interference oscillations in dissociation probabilities are discussed and these oscillations demonstrated. Intensity‐dependent line shapes for dissociation rate are obtained for various other combinations of fields. Long‐time kinetic‐energy distribution of photofragments is similar to the energy spectrum of photoelectrons obtained from atomic autoionization. The photofragment spectra for two energies in two final electronic continuum states are compared for different values of the field amplitudes and detunings.

Calculations of Einstein A coefficients for transitions between vibrational levels in NO(a ^{4}Π)
View Description Hide DescriptionThe dipole moment function calculated by Langhoff et al. [J. Mol. Spectrosc. 138, 123 (1989)] for NO(a ^{4}Π) has been used in conjunction with an RKR potential to obtain the vibrational transition probabilities of this state, for levels v=1–19. The probabilities increase with v, from 5 s^{−1} for v=1 to a maximum of 34 s^{−1} for v=13. Spectral intensities have been calculated for the situation that a ^{4}Π molecules are created at the top of the potential, and cascade down without collisional quenching. Inclusion of a radiative loss in the a–X system results in the accentuation of overtone 1100–2000 cm^{−1} emission in the spectrum. These infrared bands may be significant in an N+O recombination environment, such as afterglows and airglow.

Vibrationally induced rotational axis switching: A novel mechanism for vibrational mode coupling
View Description Hide DescriptionHigh resolution IR spectra of small‐ to medium‐sized molecules such as 2‐fluoroethanol (2FE) show that the effective density of coupled states is often greater than that obtained by a direct count of vibrational states. A novel mechanism for rotation–vibration interaction, vibrationally induced rotational axis switching (VIRAS), is proposed as a possible explanation for these discrepancies. VIRAS has its origin in centrifugal distortion, and is physically distinct from Coriolis coupling. In the case of 2FE, we explicitly treat the coupling of overall rotation with large‐amplitude internal rotation about the C–C bond. Assuming a uniform coupling of all dark vibration–torsion states to the bright state, we predict a density of coupled states in good agreement with that observed in the C–H stretching region at 2980 cm^{−1}.

Two‐photon spectroscopy of the F ^{1}Π_{ g } and f ^{3}Π_{ g } states of molecular fluorine
View Description Hide DescriptionWe report what is to our knowledge the first two‐photon excitation spectroscopy of molecular fluorine. The F ^{1}Π_{ g } and f ^{3}Π_{ g } states are excited with two photons in the range of 206–212 nm. Detection is through vacuum ultraviolet (VUV)fluorescence or ionization. Measurement of the fluorescencespectrum with a VUV spectrometer indicates that the VUVfluorescence occurs on the 157 nm F_{2} laser transition frequency. Using our spectra and the data of Porter [J. Chem. Phys. 48, 2071 (1968)] we report improved spectroscopic constants for vibrational levels in the F, f, and I ^{1}Σ_{ u } ^{+} states.

Relaxation of the first B _{ u } excited state in linear polyenes: From trans‐butadiene to polyacetylene
View Description Hide DescriptionWe present a multiphonon approach to analyze the lattice relaxation in the first B _{ u }excited state of linear polyenes, from trans‐butadiene to polyacetylene. Within a single‐electron theory including bond‐length bond‐order relationship, we optimize the geometries of both the ground state and the first excited state and characterize the geometry of the excited state in terms of the formation of soliton–antisoliton pairs (bipolarons). The vibrational modes are simply taken as corresponding to soliton translation and amplitude vibrations (dimerization modes) within the harmonic approximation. Photoabsorption experiments carried out on a series of linear polyenes, β‐carotene, polyenals/polyenones, and polyacetylene are discussed. The Huang–Rhys factor S and the relaxation energy are calculated for polyenes with various lengths. A major result is that we find that the S value first tends to decrease with increasing chain length but after some critical length the evolution completely modifies and S starts increasing. This feature is in good agreement with the experimental data and, in the case of polyacetylene, we are able to reproduce properly the experimental subgap absorption tail. The vibronic behavior for short chains is thus markedly different from that of long chains. We therefore caution that extrapolations based on observations on short polyenes might be invalid for the polymer.

Infrared spectroscopy and equilibrium structure of H_{2}O^{+}(X̃ ^{2} B _{1})
View Description Hide DescriptionA color center laserspectrometer along with velocity modulation detection was used to record the absorptionspectrum of H_{2}O^{+} produced in an AC glow discharge between 3180 and 3390 cm^{−1} with a gas mixture of He/H_{2}O. The predominant role of the reaction of metastable helium with H_{2}O has been observed. The H_{2}O^{+} ion is also present in the spectra recorded between 3100 and 3600 cm^{−1} with a gas mixture of He/H_{2}/O_{2} and some features are illustrated. The ν_{1} and ν_{2}+ν_{3}−ν_{2} bands have been assigned and the previous analysis of the ν_{3} band [J. Mol. Spectrosc. 127, 1 (1988)] has been extended. The ν_{1} and ν_{3} states have been fitted together taking into account the vibration–rotation interaction. The molecular constants have been obtained for the ν_{1} and ν_{2}+ν_{3} states and they have been improved in the case of the ν_{3} and the ground vibrational states. The equilibrium structure has been derived [r _{ e }=0.9992(6) Å, θ_{ e }=109.30(10)°] and the quadratic and cubic force field constants have been evaluated.

Spectroscopic interrogation of heterocluster isomerization. I. Simulations of nuclear dynamics and electronic spectroscopy
View Description Hide DescriptionIn this paper we explore the structure, energetics, nuclear dynamics,isomerization, and electronic‐vibrational spectroscopy of large finite systems by constant energy molecular dynamics (MD) simulations of heteroclusters of 9,10‐dichloroanthracene (DCA) with rare‐gas atoms, i.e., DCA⋅Ar_{ n } (n=1–34) and DCA⋅Kr_{ n } (n=1–18). We consider the spectroscopic implications of heterocluster size effects, isomer‐specificity, nuclear dynamics, and isomerization as manifested by the spectral shifts (δν) and homogeneous linewidths (Δ) for the S _{0}→S _{1} transition, the ionization potential shifts (δI) from S _{0}, and the shifts (Δτ_{ r }) of the pure radiative lifetimes of S _{1} for these heteroclusters, with δν, δI, and Δτ_{ r } being given relative to the corresponding observable for the bare DCA molecule. MD simulations of the energetics, rigid and nonrigid structures, fluctuations in structural parameters, and transport properties over a broad temperature region (8–70 K) allowed for the identification of several isomerization phenomena, i.e., correlated surface motion, surface melting, side crossing, wetting–nonwetting, and nearly rigid–nonrigid transitions. The size dependence of the temperature onsets for various isomerization processes was established. The hierarchical occurrence of several isomerization processes for a single heterocluster composition, together with the sequential occurrence of distinct isomerization processes with increasing the cluster size, was documented.
In the size domain explored herein the temperature onsets of surface melting, side crossing, and wetting–nonwetting transitions tend to decrease with increasing the heterocluster size, in contradiction to the rule that the cluster melting temperature increases with increasing its size, reflecting finite microsurface effects. The spectroscopic observables were simulated using quantum mechanical expressions for δν, δI, and Δτ_{ r } at a fixed nuclear configuration, and were summed over the entire MD trajectory. The heterocluster isomerization phenomena could not be identified by the temperature and/or size dependence of δν, Δ, and δI, while Δτ_{ r } data exhibit a marked structural sensitivity, providing a promising approach for the characterization of heterocluster isomerization. The MD simulations elucidate the size dependence and isomer specificity of spectral shifts, homogeneous linewidths,ionization potential shifts, and pure radiative lifetimes of heteroclusters. The intriguing phenomenon of spectral line narrowing with increasing the cluster size for these heteroclusters, is attributed to the size dependent reduction of inhomogeneous broadening which originates from the cumulative contribution of the isomer‐specific spectral shifts and of homogeneous broadening effects.

Spectroscopic interrogation of heterocluster isomerization. II. Spectroscopy of (9,10 dichloroanthracene)⋅(rare gas)_{ n } heteroclusters
View Description Hide DescriptionIn this paper we report on the electronic spectroscopy of mass‐resolved heteroclusters of 9,10‐dichloroanthracene (DCA) with rare‐gas atoms, i.e., DCA⋅Ar_{ n } (n=1–54), DCA⋅Kr_{ n } (n=1–20), and DCA⋅Xe_{ n } (n=1–14), which were interrogated by resonant two‐photon two‐color ionization in conjunction with mass spectrometric detection. We measured the spectral shifts δν and the linewidths (FWHM) Δ of the S _{0}→S _{1} transition, the ionization potential shifts δI from S _{0}, and the change Δτ_{ r } in the pure radiative lifetime of S _{1}, relative to the corresponding values for the bare DCA molecule. We were able to provide a global assignment of the structure of isomers of DCA⋅A_{ n } (A=Ar,Kr) heteroclusters, which rests on six independent sources of information, i.e., the application of experimental combination rules and on the comparison between the results of the molecular dynamics (MD) simulations and the experimental data for δν, δI, and Δτ_{ r }. The size dependence and the isomer specificity of δν, δI, and Δτ_{ r } are well accounted for by microscopic theories for these spectroscopic observables in conjunction with MD simulations. The size dependence of the experimental linewidths was analyzed in terms of finite temperature MD simulations, which incorporate the effects of homogeneous and inhomogeneous line broadening. Our analysis provides an adequate description of the qualitative difference between the gradual line narrowing of DCA⋅Ar_{ n } (n=20–34) and the abrupt onset of line narrowing of DCA⋅Kr_{ n } (n≥16) heteroclusters, which are attributed to the reduction of inhomogeneous line broadening originating from the combined effects of homogeneous line broadening and of the isomer specificity of the spectral shifts.
The temperature dependence of the homogeneous linewidths provides means for internal cluster thermometry. The isomerization phenomena inferred from MD simulations of nuclear dynamics could not be identified by the cluster size dependence of δν, δI, Δτ_{ r }, and Δ, in accord with the conclusions drawn from the MD simulations of these spectroscopic observables. Finally, we explored the evolution of condensed matter properties with increasing the heterocluster size, advancing cluster size equations, which quantitatively accounts for the smooth ‘‘transition’’ of the spectral shift and of the homogeneous line broadening of DCA⋅Ar_{ n } heteroclusters to those of DCA in bulk solid Ar.

Electronic spectra of jet‐cooled 5‐bromotropolone and 5‐chlorotropolone. Influence of symmetrical substitution on proton tunneling in the S _{1} state
View Description Hide DescriptionThe S _{1}–S _{0}fluorescence excitation and dispersed fluorescence spectra have been measured for jet‐cooled 5‐bromotropolone (5BTR‐h), 5‐chlorotropolone (5CTR‐h), and their OD derivatives in order to investigate substituent effects on protontunneling. Both the 0_{+} ^{+} and 0_{−} ^{−} transitions were identified in the fluorescence spectra of these molecules. The tunneling doublet splittings in the electronic origin band of 5BTR‐h (16 cm^{−1}) decreased by 4 cm^{−1}, whereas that of 5CTR‐h (23 cm^{−1}) increased by 3 cm^{−1} as compared with tropolone (TRN‐h). Relatively small deviation in the tunneling separations for these molecules from that for tropolone is ascribed to conjugation in the S _{1} state. An electron‐withdrawing character of chlorine and bromine atoms is almost canceled by a conjugative electron‐releasing effect.

Cross relaxation in magnetic resonance: An extension of the Solomon equations for a consistent description of saturation
View Description Hide DescriptionIn magnetic resonance studies of cross relaxation or Overhauser effects by steady‐state continuous irradiation, it is shown that the Solomon equations do not allow one to obtain a consistent picture of the expectation values of various magnetization components. The contradictions can be resolved if, in addition to the Zeemanpolarizations, one takes into account a transverse magnetization component that is associated with the spin which is being irradiated.

Intermolecular HF motion in Ar_{ n }HF micromatrices (n=1,2,3,4): Classical and quantum calculations on a pairwise additive potential surface
View Description Hide DescriptionThe availability of pairwise additive ‘‘two‐body’’ potentials for van der Waals systems from near‐IR, far‐IR and microwave data permits detailed prediction of librational behavior for isolated HF chromophores solvated by successive numbers of rare gas Ar atoms. This paper describes theoretical calculations of Ar_{ n }HF equilibrium structures and intermolecular HF vibrational frequencies based on an Ar_{ n }HF ‘‘two‐body’’ potential energy surface developed from previously determined Ar–Ar and Ar–HF potentials. Isomeric structures are predicted from local minima on these multidimensional surfaces, and are found to be in excellent qualitative agreement with near‐IR observations of Ar_{ n }HF clusters with n=1,2,3, and 4 Ar atoms. Quantum mechanical calculations are performed for the HF librational and van der Waals stretching modes against a rigid Ar_{ n } frame. These pairwise additive potentials predict a strongly increasing angular anisotropy for the HF bending coordinate with number of Ar atoms (for small n), and provide predictions of HF intermolecular van der Waals bend and stretch vibrational frequencies. Fourier transform (FT)‐microwave and near‐IR data, on the other hand, demonstrate only a minor dependence of the anisotropy on n; this suggests the pairwise additive potentials may systematically overestimate the angular anisotropy for HF bending. Selected cuts through these potential surfaces indicate significant coupling between the Ar_{ n }–HF stretch, Ar–Ar stretch, and Ar–Ar bending vibrations. This strong vibrational coupling indicates that a full quantum treatment of all intermolecular coordinates may be required in order to make quantitative comparison with van der Waals vibrational data. In the limit of sufficient Ar atoms to fill the first coordination sphere around the HF, the calculations indicate a nearly perfect cancellation of angular anisotropy for HF librational motion, consistent with the nearly free internal rotation of the HF observed in cryogenic Ar matrices.

Spectroscopy and relaxation dynamics of I_{2}Ar_{ n } clusters. Geminate recombination and cluster fragmentation
View Description Hide DescriptionI_{2}Ar_{ n } clusters yield visible and near‐UV emissions when excited to the states which correlate with the first ion‐pair manifold of I_{2}. These states may be accessed by 193 nm excitation of ground‐state clusters, or near‐UV excitation of electronically metastable I_{2}(A’ or A)Ar_{ m }. Comparisons of the cluster and I_{2}/Ar matrix spectra suggest that such excitations result in fragmentation and ‘‘melting’’ of the clusters prior to emission. 532 nm photodissociation of I_{2} within the clusters is followed by geminate recombination which populates the A’, A, and X states. The probability for ejection of the recombined I_{2} from the clusters during the relaxation process appears to be size dependent. These results represent tentative steps towards the study of I(^{2} P _{3/2})+I(^{2} P _{3/2}) recombination in Ar clusters. They also indicate that dissociation, recombination, and fragmentation processes may be used as a general method for generating metastable species in supersonic expansions.

Electric field splitting of the octatetraene 1 ^{1} A _{ g }→2 ^{1} A _{ g } transition in n‐hexane
View Description Hide DescriptionOctatetraene can be photoisomerized even when it is incorporated in a low temperature n‐hexane crystal. When this is done by irradiating the zero‐phonon component of the S _{0}→S _{1} origin band with a single frequency laser, very narrow (∼10 MHz) persistent holes can be burned. We have determined the effect of an externally applied electric field on these persistent holes and fit the observed data with a simple quantum mechanical model parameterized in the S _{0}→S _{2} and S _{1}→S _{2} transition dipoles and the magnitude of a molecular field intrinsic to the n‐hexane site occupied by the octatetraene guest. The magnitude of the molecular field (∼10^{6} V/cm) is consistent with only one of the three possible n‐hexane sites that could have accommodated the octatetraene guest.

Spectroscopy and dynamics of jet‐cooled hydrazines and ammonia. I. Single‐photon absorption and ionization spectra
View Description Hide DescriptionElectronic‐state properties of hydrazine (N_{2}H_{4}), monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH), and ammonia were investigated by single‐photon vacuum ultraviolet (vuv)absorption and photoionizationspectroscopy in a molecular‐beam apparatus. The photoionizationspectrum of NH_{3} shows a sharp threshold at a value of 10.16 eV in excellent agreement with previous measurements. The ionization thresholds of the hydrazines are very broad reflecting the large (nearly 2 eV) structural relaxation energy for conversion from a C _{2} pyramidal gauche neutral geometry to a D _{2h } planar ion geometry. The measured threshold photoionization potentials are 8.32 eV (N_{2}H_{4}), 8.05 eV (MMH), and 7.87 eV (UDMH). The vuvabsorption spectra of the hydrazines are also broad, which is to be contrasted with the well‐resolved spectrum of NH_{3}. The band shapes can be attributed to planarization of the Rydberg excited states in analogy to the ion structure. A molecular‐orbital model is developed to provide understanding of the excited states in terms of their symmetries, energy ordering, and selection rules, as a function of geometry.

Spectroscopy and dynamics of jet‐cooled hydrazines and ammonia. II. Electron‐impact dissociative ionization
View Description Hide DescriptionDissociative‐ionization cross sections, fragment appearance potentials, and fragment kinetic energies were measured for electron‐impact excitation of jet‐cooled NH_{3}, hydrazine (N_{2}H_{4}), and monomethyl hydrazine (MMH) over an energy range of 10–270 eV. A data base of 35 parent and fragment ions is reported. All measurements were made in a crossed electron–molecular beam apparatus using pulsed extraction and time‐of‐flight mass detection to ensure field‐free excitation and high collection efficiency for energetic ions. Cross sections for NH_{3}ionization are in good agreement with previous measurements except for ions with high kinetic energy (KE). These discrepancies are attributed to instrument‐dependent KE detection efficiencies in the previous results. Cross section data have not been previously reported for N_{2}H_{4} and MMH. The measured cross sections for total ionization at 70 eV are 2.35 Å^{2} (NH_{3}), 3.76 Å^{2} (N_{2}H_{4}), and 4.20 Å^{2} (MMH). KE distributions were measured by an ion deflection method and gave results consistent with time‐of‐flight peak‐shape analysis. Mean KE values <ε_{ t }≳ are reported for all fragment ions studied. For 170‐eV excitation of NH_{3}, <ε_{ t }≳ varied from 0.026 eV (NH_{2} ^{+}) to 1.4 eV (H^{+}). The kinetic energies for N_{2}H_{4} and MMH fragment ions at similar excitation energies are typically much lower than for fragment ions from NH_{3}, conforming to statistical arguments based on density of internal states. High resolution mass spectra were recorded for MMH in order to distinguish different fragment ions of the same unit mass. Substantial rearrangement is evident for N_{2}H_{4} and MMH dissociativeionization based on the appearance of ions such as NH_{3} ^{+} and NH_{4} ^{+} (the latter for MMH ionization only) and the magnitude of <ε_{ t }≳ for certain ions. The role of electronic structure and geometry on dissociation is explored using a molecular orbital analysis to predict product correlations for the excited states of N_{2}H_{4} ^{+}.

On the dissociation energy of O_{2} and the energy of the O_{2} ^{+} b ^{4}Σ_{ g } ^{−} state
View Description Hide DescriptionWe note here that a significant discrepancy exists between the recently determined dissociation energy of the O_{2} B ^{3}Σ_{ u } ^{−} state and the energy of the O(^{1} D _{2})+O(^{3} P _{2}) dissociation limit determined in a spectroscopic cycle. The major contribution to this discrepancy appears to be the currently accepted value for the energy of the O_{2} ^{+} b ^{4}Σ_{ g } ^{−} state relative to the ground electronic state of molecular oxygen.

Theoretical and experimental structures of vinyl fluoride and vinyl alcohol
View Description Hide DescriptionA new theoretical r _{0} structure for vinyl fluoride has been derived using ab initio molecular orbital procedures. In addition, new experimental r _{0} and r _{ s } structures have been obtained through a reanalysis of existing experimental microwave data. Significant discrepancies are found between the new theoretical (125.4°) and experimental (127.6°) r _{0} estimates of the CCH_{ u } angle involving the hydrogen atom geminal to the substituted center. This is attributed to the relative insensitivity of the experimental structural fits to the precise value of this bond angle, an insensitivity that is associated with the proximity of the relevant atoms to inertial axes. Because of these difficulties, the best estimate of the experimental r _{0} structure for vinyl fluoride is obtained by taking the theoretical r _{0} value for the CCH_{ u } angle and determining the remaining parameters by a weighted‐least‐squares fit to the experimental data. Weighted‐least‐squares fits are found generally to provide more consistency in the derived structural parameters than unweighted procedures. It is found that fitting of isotopic differences in rotational constants (as opposed to the conventional fitting of rotational constants themselves) provides the structure for vinyl fluoride most similar to the theoretical r _{0} structure. Similar observations are found to hold for vinyl alcohol, for which new theoretical and experimental results are presented.

Study of C_{6} ^{−} and C_{6} with threshold photodetachment spectroscopy and autodetachment spectroscopy
View Description Hide DescriptionThe C_{6} ^{−} anion and C_{6} neutral have been studied using both threshold photodetachment (zero electron kinetic energy) spectroscopy and autodetachment spectroscopy of C_{6} ^{−}. The threshold photodetachment spectrum yields the electron affinity of linear C_{6} to high accuracy, along with the three symmetric stretch frequencies for linear C_{6} and the spin–orbit splitting in the ground ^{2}Π_{ u } state of the anion. Two of the symmetric stretch frequencies are significantly lower than previous ab initio predictions. A simple model force field is used to calculate stretching force constants and estimate bond length changes between the anion and neutral. In addition, using autodetachment spectroscopy, we have located an excited electronic state of C_{6} ^{−} that lies 43 cm^{−1} below the detachment threshold. This state is very similar in geometry to neutral C_{6}. Excited vibrational levels of this state autodetach with rates that depend strongly on the available autodetachment channels. The excited state is tentatively assigned to a valence state, rather than an electrostatically bound state.

Non‐Lorentzian depolarized Raman line shapes in n‐pentanol
View Description Hide DescriptionHigh resolution depolarized Raman spectra of n‐pentanol in the −30 to 60° C temperature range are reported. The spectra appear as a continuously decreasing line shape centered at zero frequency. A quantitative analysis of this line shape shows a high frequency exponential tail, which can safely be assigned to intermolecular collision phenomena, and a low frequency narrow contribution. The line shapes of the low frequency contribution are seen to deviate from a single Lorentzian, the effect being more pronounced as the temperature is increased. The data have been analyzed assuming the non‐Lorentzian line shape as due either to a distribution of Debye relaxation times or to a single non‐Debye relaxation mechanism. The analysis in terms of a distribution of relaxation times suggests a broadening of the distribution as the temperature increases, while the ‘‘average’’ relaxation time shows an Ahrenius behavior which compares well with the one observed in the ultrasonic attenuation measurements. Similar results have been obtained considering the nonexponential relaxation mechanisms. Absolute intensities are also evaluated with respect to the Brillouin doublet and discussed in terms of the possible microscopic scattering mechanisms.

Identification of the nd Δ and Σ states and the ^{1,3}Φ←←X ^{3}Σ^{−} _{ g } transition of O_{2} by resonant multiphoton ionization
View Description Hide DescriptionSpectra of the 3dRydberg state region of O_{2} have been obtained by two‐photon resonant ionization of the ground electronic state. By varying the rotational distribution and radiation polarization, all observed bands were identified and attributed to excitation of Σ, Δ, and Φ states. Earlier assignments were corrected. The Δ and Φ assignments are complete while the Σ assignments are so far incomplete.