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Volume 97, Issue 7, 01 October 1992

Study of Renner–Teller, spin–orbit, and Fermi‐resonance interactions in X̃ ^{2}Π (v _{1} v _{2}0) levels of NCO by stimulated emission pumping spectroscopy
View Description Hide DescriptionWe report extensive measurements of rovibronic levels in the 020/100 and 040/120/200 Fermi‐resonant polyads of NCO. The levels were accessed by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of bands in the Ã ^{2}Σ^{+}−X̃ ^{2}Π spectrum of the radical. The data were analyzed in terms of an effective Hamiltonian which explicitly included reference to all the possible vibronic levels of ^{2}Π symmetry in the polyad under consideration. The effects of levels outside these were treated by perturbation theory. The model was successful in fitting the data close to the experimental measurement precision and the resulting parameters were interpreted in terms of the harmonic and anharmonic vibrational terms. The effective Renner–Teller coupling parameter in NCO varied with vibrational level; however, parameters describing the Fermi‐resonance interaction were found to be constant for all the levels investigated.

Isothermal density tuning of the depolarization ratios from the ν_{1} mode of methane
View Description Hide DescriptionPure methane and 10 mol % methane in Ar have been studied in the region of methane’s ν_{1} totally symmetric stretching mode near 2910 cm^{−1} (ω_{0}, the signal maximum or zeroth spectral moment frequency). Densities from 0.0795 (3 bar) to 15.0 (2100 bar)×10^{21} mol/c.c. were examined using Ar^{+} laser illumination at 514.5 nm. Frequency shifts of ν_{1}’s ω_{0}, and the zeroth moment depolarization ratio, ρ_{ D }, values as a function of pressure were recorded. Previous ω_{0} shifts vs density were generally confirmed and were extended to higher densities. This allowed a shallow minimum to be observed in this data. It was correlated with the ρ_{ D } values. These latter values were examined to see whether or not they were as sensitive as the frequency shift results were to the local reorientational friction coefficient tensorf _{ r } (potential‐energy interactions) in methane and the Ar/CH_{4} solutions. The initial (3.5–1000 bar data) depolarization results were interpreted with two models of methane in mind. The first, a conventional view that is based upon the concept that higher densities cause distortions in the molecular shape (lower symmetry), and the second, which makes use of the known fact that the symmetry of methane at low pressures is less than T _{ d }. Therefore, as the density and f _{ r } increase quantum rotation slows and the rotation‐vibration interaction effectively ceases and causes the molecules to pack tightly and the well‐known T _{ d } symmetry is achieved.

Spectroscopic studies of jet‐cooled NiAu and PtCu
View Description Hide DescriptionSpectroscopic investigations of NiAu and PtCu have revealed that both molecules possess ^{2}Δ_{5/2}ground electronic states, and are in this respect analogous to the isovalent molecule NiCu. The ground‐state bond lengths (r _{0}) have been measured as 2.351±0.001 Å and 2.335±0.001 Å for NiAu and PtCu, respectively. Ionization potentials have been bracketed as well, giving IP(NiAu)=8.33±0.38 eV and IP(PtCu)=8.26±0.07 eV. A reanalysis of previous high‐temperature Knudsen effusion mass spectrometric data provides D _{0} ^{0}(NiAu)=2.52±0.17 eV. The implications of these results for the electronic structure and chemical bonding of NiAu and PtCu are discussed, and comparison is made to the other diatomic metals of the nickel and copper groups.

The relation between intensity and dipole moment for bending modes in linear molecules
View Description Hide DescriptionBy careful summation over the rotational fine structure, we examine the relation between dipole moment derivative and intensity of bending modes in linear molecules. The relation is derived without the usual assumption of the harmonic oscillator approximation and it is emphasized that the intensity of the degenerate mode cannot be viewed simply as a sum of two orthogonal one‐dimensional oscillators, although the intensity is found to be close to two times the intensity of a nondegenerate mode with the same dipole moment derivative. The importance of using the Γ intensity rather than the S intensity for the total band intensity is demonstrated, and finally the effect of the Coriolis coupling is summarized. It is pointed out that most recent quantum mechanical computations predict band‐intensities of the bending modes in linear molecules that are approximately a factor of 2 bigger than the observed intensities, and it is shown that the problem relates to the ab initio treatment.

Line‐by‐line measurements of interference parameters for the 0–1 and 0–2 bands of CO in He, and comparison with coupled‐states calculations
View Description Hide DescriptionHere we present the first line‐by‐line measurements of interference parameters Y ^{0} _{ k } describing line‐mixing effects in the weak overlapping regime for He‐broadened CO lines in the 0–1 and 0–2 bands at 296 K. A detailed analysis of the line shape at intermediate perturber pressures (up to about 10 atm) has been performed, starting from previous theoretical calculations, which has demonstrated the possibility of an individual measurement of Y _{ k } parameters. The method is based on the existence of a component of the line shift, quadratic with the perturber pressure (density) and proportional to Y ^{0} _{ k }. Comparison of our measurements with results predicted from coupled‐states calculations shows good overall agreement. Linear pressure shifts have also been simultaneously measured. The uncertainty on the data is important since linear shifts are very small; however, some features can be considered as significant although we have no clear understanding of them. Possible explanations are discussed.

The 3d _{Ni} ^{8}(^{3} F)3d _{Cu} ^{10}σ^{2}σ*^{1} manifold of excited electronic states of NiCu
View Description Hide DescriptionDiatomic NiCu has been supersonically cooled in a molecular beam and investigated by resonant two‐photon ionization spectroscopy. A total of nine band systems connecting the ground 3d ^{9} _{Ni}3d ^{10} _{Cu}σ^{2}, X ^{2}Δ_{5/2} state to the 3d ^{8} _{Ni}(^{3} F)3d ^{10} _{Cu}σ^{2}σ*^{1}manifold of states have been found, and bands of eight of these systems have been rotationally resolved and analyzed. L‐ and S‐uncoupling interactions have been found between two pairs of states, resulting in B[11.9]2.5∼C[11.9]1.5 and [10.4]2.5∼[10.4]1.5(?) heterogeneous perturbations. In the case of the B∼C interaction, the magnitude of the perturbation matrix element has been deduced. The detailed spectroscopicanalysis of the 3d ^{8} _{Ni}(^{3} F)3d ^{10} _{Cu}σ^{2}σ*^{1}manifold of states presented in this paper allows the electronic structure of NiCu to be understood in depth and provides a database for comparison to the ligand‐field plus spin–orbit calculation of the NiCu excited states presented in the following paper.

Ligand‐field theory applied to diatomic transition metals. Results for the d _{ A } ^{9} d _{ B } ^{9}σ^{2} states of Ni_{2}, the d _{Ni} ^{9} d _{Cu} ^{10}σ^{2} states of NiCu, and the d _{Ni} ^{8}(^{3} F)d _{Cu} ^{10}σ^{2}σ*^{1} excited states of NiCu
View Description Hide DescriptionA ligand‐field theory has been developed for transition‐metal diatomics having electronic configurations of d _{ A } ^{9} d _{ B } ^{10}σ^{2}, d _{ A } ^{9} d _{ B } ^{9}σ^{2}, and d _{ A } ^{8}(^{3} F)d _{ B } ^{10}σ^{2}σ*^{1}. The theory treats each atom as a point charge and includes spin–orbit interactions. No contributions due to d‐orbital chemical bonding are included. Since the d orbitals are quite small compared to the bond lengths in these molecules, the only inputs to the theory are the ligand charges (Z _{ A } and Z _{ B }), the radial expectation values <r _{ A } ^{2}≳_{ nd }, <r _{ B } ^{2}≳_{ nd }, <r _{ A } ^{4}≳_{ nd }, and <r _{ B } ^{4}≳_{ nd }, the atomic spin–orbit parameters ζ_{ A } and ζ_{ B }, and the bond length, R. Calculations employing no adjustable parameters (setting Z _{ A, B } =+1.0, and using radial expectation values and spin–orbit parameters from atomic tables) provide essentially quantitative agreement with ab initio results on the d _{Ni} ^{9} d _{Cu} ^{10}σ^{2}manifold of states in NiCu, and on the d _{ A } ^{9} d _{ B } ^{9}σ^{2}manifold of states in Ni_{2}.
This demonstrates that the ligand‐field model has some validity for metal molecules containing nickel, primarily because of the compact nature of the 3d orbitals in this element. Similar calculations of the d _{ A } ^{9} d _{ B } ^{9}σ^{2}manifold of states in Pt_{2} and the d _{Ni} ^{9} d _{Pt} ^{9}σ^{2}manifold of states in NiPt are presented for comparison to future ab initio or experimental measurements, although the possibility of d‐orbital contributions to the bonding in these species makes the ligand‐field model less favorable in these examples. The d _{Ni} ^{8}(^{3} F)d _{Cu} ^{10}σ^{2}σ*^{1}excited electronic states of NiCu, which are well known from resonant two‐photon ionization spectroscopy, are also investigated in the ligand‐field model. As a final example, the d _{Ni} ^{8}(^{3} F)σ^{2}σ*^{1}excited electronic states of NiH are also examined using the same treatment as that employed for the d _{Ni} ^{8}(^{3} F)d _{Cu} ^{10}σ^{2}σ*^{1} excited manifold of NiCu.

Spectroscopic study of Λ‐doublet, spin, and rotational relaxation in the NH(A ^{3}Π,v=0) state
View Description Hide DescriptionElectronically excited NH(A ^{3}Π) radicals in single N’, J’, e/f states were investigated by pumping on isolated NH(A←X) lines of the (0,0) band. Collision‐induced transitions among the different Λ‐doublet, spin, and rotational states were monitored by fluorescence spectra. In collisions with NH_{3}, a propensity for conservation of spin is observed. In the original spin unit, rotational relaxation occurs preferably to the neighboring rotational levels. The efficiency of spin‐unit changes decrease with increasing ΔΩ. For NH_{3} collisions inducing a fine‐structure change, the rotational distribution is found to be thermal and no memory of the original rotational level is left. In collisions with Ar, spin is not conserved. Generally, relaxation into states of the same Λ‐doublet component occurs with approximately the same probability as into the other component. Relaxation processes induced by Ar are less efficient than those caused by collisions with NH_{3}.

Molecular‐beam laser spectroscopy of lanthanum monofluoride: Magnetic hyperfine and dipole moment measurements
View Description Hide DescriptionThe lanthanum monofluoride molecule (LaF) was generated in a pulsed molecular beam by chemical reaction in a laser‐produced plasma. The (0,0) and (1,0) bands of the B ^{1}Π–X ^{1}Σ^{+} system of LaF (ν_{00}=16 184.52 cm^{−1}), and the 0^{+}–X ^{1}Σ^{+} band (ν_{00}=16 637.95 cm^{−1}), were investigated at sub‐Doppler resolution (120 MHz) using a ring dye laser to excite fluorescence. The electron orbital‐nuclear spin interaction parameter (magnetic hyperfinea parameter) was determined to be +138(5) MHz and +149(5) MHz for the v=0 and v=1 levels of the B ^{1}Π state, respectively (2σ error bounds). The observed hyperfine structure is interpreted in terms of ligand field theory. Molecular rotational constants for all three bands were found to be in good agreement with previous work [Schall et al., J. Mol. Spectrosc. 100, 437 (1983)]. The permanent electric dipole moments of the X ^{1}Σ^{+} and 0^{+} states of LaF were determined by molecular‐beam Stark spectroscopy to be 1.808(21) D and 3.43(10) D (2σ errors). Results are compared with recent experimental determinations of the dipole moments of the other group III monofluorides.

Laser threshold photoelectron spectra of the cis and trans rotational isomers of p‐dimethoxybenzene–Ar_{ n } (n=0,1,2): Observation of the intermolecular van der Waals stretching and bending vibrational modes in the cation
View Description Hide DescriptionThe cis and trans rotational isomers of p‐dimethoxybenzene–Ar_{ n } (n=0,1,2) have been studied in a supersonic free jet by two‐color laser resonance enhanced multiphoton ionization threshold photoelectron spectroscopy. The two‐color (1+1’) threshold photoelectron spectra recorded via the S _{1} state of the cis and trans isomers of the 1:1 and 1:2 argon complexes reveal well resolved vibrational structure characteristic of the low frequency bending and stretching van der Waals vibrational modes. In the case of the trans isomer of the 1:2 complex, a very low frequency progression (11 cm^{−1}) in a nontotally symmetric van der Waals bending mode appears in single quanta in the spectrum. The equivalent spectrum recorded for the cis isomer exhibits structure characteristic of van der Waals stretching modes as well as double quanta excitation in both totally symmetric and nontotally symmetric van der Waals bending modes. The observation of single quantum excitation in formally forbidden van der Waals vibrational modes implies the possibility of a change in the overall symmetry of the complex in the ground cationic state when compared to that in the S _{1} state. The adiabatic ionization energies (I _{ a }) for the cis and trans isomers of p‐dimethoxybenzene–Ar_{ n } (n=0,1,2) were measured as 60 774±7 (cis; n=0), 60 687±7 (cis; n=1), 60 509±7 (cis; n=2), 60 563±7 (trans; n=0), 60 479±7 (trans; n=1), and 60 295±7 cm^{−1} (trans; n=2).

Localization of a continuum shape resonance. Photoionization of CS_{2}
View Description Hide DescriptionWe report a vibrationally resolved investigation into the 5σ_{ u } ^{−1} shape‐resonant ionization dynamics for CS_{2} in the range 18≤hν≤30 eV. The intensity of dispersed fluorescence from CS_{2} ^{+}(B ^{2}Σ_{ u } ^{+}) photoions is measured to obtain partial photoionization cross‐section curves for the v=(0,0,0) and (1,0,0) levels of CS_{2} ^{+}(B ^{2}Σ_{ u } ^{+}), as well as the vibrational branching ratio. Our results indicate a shape resonance at hν≊21 eV which is insensitive to changes in the symmetric stretching coordinate. These data are consistent with recent theoretical efforts that predict a shape resonance in the 5σ_{ u }→επ_{ g } channel. All previous vibrationally resolved data on shape resonances have been obtained for systems whose shape resonances occur in the εσ continuum. The current results are in contrast to behavior observed for other shape resonances, highlighting both their diverse nature and possible extensions of the current measurements.

Xenon enhancement and third harmonic generated lines in the 3+m‐photon ionization spectroscopy of CO (A ^{1}Π←X ^{1}Σ^{+}) in CO+Xe mixtures
View Description Hide DescriptionA study of 3+m‐photon ionization spectra of CO (A←X) transitions in CO+Xe mixtures shows strong Xe enhancement and the appearance of extra ionization ‘‘lines’’ to the violet end of the respective CO (A←X) vibrational bands. Both facts are adequately interpreted as the result of reabsorption of laser‐induced third harmonic generation (THG) in CO+Xe mixtures. The shift of the THG line from the CO (A←X) band origin and its profile, together with their dependence on the CO/Xe pressure ratio, have all been satisfactorily predicted by calculations based on the phase‐matching requirement for THG, in which a semiempirical expression for the refractive index of CO is proposed and used with success.

Nonlinear polarization description of phase‐locked pulse‐pair spectroscopy
View Description Hide DescriptionThe recently demonstrated technique of optically phase‐locked pulse‐pair (PLPP) excited spontaneous emission is described by a third‐order perturbative density matrix approach. A nonlinear polarization description shows how PLPP spectroscopy depends on all the relevant material dephasing time scales. The time and frequency integrated resonance spontaneous emission consists entirely of resonance fluorescence, and is derived exclusively from excited‐state population decay terms, i.e., diagonal second‐order density‐matrix elements. These third‐order polarization results are proportional to the previously derived linear polarization expressions found to describe the observed PLPP I _{2} vapor emission. The nonlinear treatment allows a comparison of this technique to other forms of ultrafast pump–probe spectroscopies such as transient absorption and photon echo techniques. The role of impulsively prepared coherences is clearly described by this analysis. The effect of pulse duration, relative to material dephasing times, is explored for relaxation given by the optical Bloch equations. The most significant differences between a linear and nonlinear polarization treatment occur for pulse durations greater than the optical dephasing time or when excited state population and coherence decays are of the order of rovibrational periods. A fluorescence line narrowing effect, due to short pulse excitation, is predicted.

Spin–orbit perturbations between the A(2)^{1}Σ^{+} and b(1)^{3}Π_{0} states of NaK
View Description Hide DescriptionWe have studied spin–orbit perturbations between the A(2) ^{1}Σ^{+} and b(1) ^{3}Π_{0} states of the NaK molecule by accurately measuring the energies of mutually perturbing levels, and by measuring ratios of A(2) ^{1}Σ^{+} → X(1) ^{1}Σ^{+} and b(1) ^{3}Π_{0} → a(1) ^{3}Σ^{+} emission intensities, for five perturbed pairs. This allows two partially independent determinations of each perturbation matrix element ‖〈1 ^{3}Π_{0}(v ^{’} _{ t }, J’)‖Ĥ_{so}‖2^{1}Σ^{+}(v ^{’} _{ s },J’)〉‖. From these matrix elements, and calculated vibrational overlap integrals 〈v ^{’} _{ t }‖v ^{’} _{ s }〉, the electronic part of the perturbation matrix element, H _{el}≡‖〈1 ^{3}Π_{0}(v ^{’} _{ t }, J’)‖Ĥ_{so}‖2 ^{1}Σ^{+}(v ^{’} _{ s }, J’)〉‖/‖〈v ^{’} _{ t }‖v ^{’} _{ s }〉‖, was obtained. Our results for H _{el} from the two methods are consistent, and independent of vibrational and rotational quantum numbers, as expected. The determined best value for H _{el} is (15.64±0.39) cm^{−1}.

Collisional cooling between 5 and 20 K: Low‐temperature helium pressure broadening of CH_{3}F
View Description Hide DescriptionA recent adaptation of the collisional cooling technique which permits gas‐phase spectroscopy from 5 K to ≥20 K is described. The new apparatus was used to measure pressure broadening parameters and cross sections for the the J=0–1, K=0 rotational transition of CH_{3}F broadened by helium from 5 to 21 K. The cross sections show a general upward trend with decreasing temperature ranging from 55.9 Å^{2} at 20 K, to 114.0 Å^{2}, at 5 K. This compares with a 295 K cross section of 48.8 Å^{2}. While an accurate CH_{3}F–He potential surface is not presently available for calculating theoretical cross sections, the rise in cross section at low temperature can be attributed to the dominance of resonant collisional processes at very low energy. Although resonant structure in many systems (e.g., CO–He, H(D)Cl–He) appears to be smoothed out by Boltzmann averaging, there is experimental evidence that at least one resonance may survive the thermal average in the CH_{3}F–He system.

Multidimensional hydrogen tunneling dynamics in the ground vibrational state of the ammonia dimer
View Description Hide DescriptionWe have measured and assigned more than 800 new far‐infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far‐infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric‐dipole‐allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy‐level diagrams for the K _{ a }=0 and K _{ a }=1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G _{144}. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the ‘‘donor’’ and ‘‘acceptor’’ roles of the monomers, internal rotation of the monomers about their C _{3} symmetry axes, and quite unexpectedly, ‘‘umbrella’’ inversion tunneling.
In the K _{ a }=0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In K _{ a }=1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm^{−1}, respectively. In the A+E states, the measured 20.5 cm^{−1} splitting can result from a difference in ‘‘donor’’ and ‘‘acceptor’’ internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far‐infrared subbands is the very‐low‐frequency out‐of‐plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that ‘‘donor–acceptor’’ interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six‐dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

Computational exploration of the six‐dimensional vibration–rotation–tunneling dynamics of (NH_{3})_{2}
View Description Hide DescriptionIn order to address the well‐known problem that the nearly cyclic structure of (NH_{3})_{2} deduced from microwave spectra differs greatly from the hydrogen‐bonded equilibrium structure obtained from ab initio calculations, we have calculated the vibration–rotation–tunneling (VRT) states of this complex, and explicitly studied the effects of vibrational averaging. The potential used is a spherical expansion of a site–site potential which was extracted from ab initio data. The six‐dimensional VRT wave functions for all the lowest states with J=0 and J=1 were expanded in products of radial (van der Waals stretch) functions and free‐rotor states for the internal and overall rotations, which were first adapted to the complete nuclear permutation inversion group G _{36}. Although the (expanded) potential is too approximate to expect quantitative agreement with the observed microwave and far‐infrared spectra, we do find several interesting features: The ^{14}N quadrupole splittings and the dipole moment of the complex, which are indicative of the orientational distributions of the NH_{3}monomers, are substantially affected by vibrational averaging. The interchange tunneling of the two monomers is not quenched. In the ortho–ortho and para–para states, of A and E symmetry, this tunneling manifests itself in a very different manner than in the ortho–para states of G symmetry. In contrast with the interpretation of Nelson et al. [J. Chem. Phys. 87, 6364 (1987)], we believe that the G _{α} and G _{β} states observed by these authors correspond to a single VRT state which is split by (hindered) NH_{3}monomer inversion.

Photoionization energetics and dynamics of gas‐phase SnCl_{2} by synchrotron radiation photoemission and configuration interaction calculations
View Description Hide DescriptionThe photoionization from outer‐ and inner‐valence shells of SnCl_{2} vapor, induced by means of molecular effusive beam technique and dispersed synchrotron radiation as an ionizing source, reveals new spectroscopic and dynamical aspects in this molecule. Spectral features related to states with main Cl 3s contribution are observed for the first time in a molecule with Cl‐metal bonds. The corresponding ionization energy (IE) is 22.61 eV. A breakdown of the one‐particle model is exhibited in the inner‐valence spectral region. These experimental findings are compared with the theoretical predictions obtained by performing configuration interaction (C.I.) calculations for the molecular ionic states. A strongly resonant behavior in the cross section of some main lines and satellites is experimentally pointed out by tuning the excitation photon energy through resonances localized at 25.03 and 26.11 eV. CI calculations for the transition energies relative to 4d excitations have been carried out and explain the aforementioned phenomena in terms of autoionizing 4d→LUMO lowest unoccupied molecular orbital 8b _{1} excitations. Finally, Auger processes following the relaxation of Sn 4d hole in the free molecule SnCl_{2} are observed.

Spectra and relaxation paths of Hg(^{3} P _{1}) in rare gas matrices
View Description Hide DescriptionAbsorption, fluorescence‐excitation and fluorescence spectra, and lifetimes have been measured for ^{3} P _{1}↔^{1} S _{0} and ^{3} P _{0}→^{1} S _{0} transitions in Hg atoms isolated in Ar, Kr, and Xe matrices. In all systems, the ^{3} P _{1} state decays mainly by fluorescence emission; the ^{3} P _{1}■^{3} P _{0} relaxation, inducing the long‐lived ^{3} P _{0}→^{1} S _{0} emission, being very inefficient. The ^{3} P _{1}→^{1} S _{0} emission in Ar and Kr is a mirror image of the absorption, while a strong redshift in Xe corresponds to the formation of Hg^{*}Xe exciplexes. The results are discussed in a model assuming the additivity of interatomic potentials deduced from the spectroscopic studies of jet‐cooled Hg–rare‐gas complexes.

A test of the possibility of calculating absorption spectra by mixed quantum‐classical methods
View Description Hide DescriptionSome of the most efficient methods for studying systems having a large number of degrees of freedom treat a few degrees of freedom quantum mechanically and the remainder classically. Here we examine how these methods fare when used to calculate the cross section for photon absorption by a quantum system imbedded in a medium. To test the method, we study a model which has two degrees of freedom and mimicks the properties of a one‐dimensional alkali atom–He dimer. We treat the electron motion quantum mechanically and the distance between the He atom and the alkali ion classically. Light absorption occurs because the electron is coupled to radiation. The calculation of the absorption cross section by quantum‐classical methods fails rather dramatically−at certain frequencies, the absorption coefficient is negative. By comparing with exact quantum calculations, we show that this failure takes place because the time evolution of the classical variables influences the dynamics of the quantum degree of freedom through the Hamiltonian only; important information, which a fully quantum treatment would put in the wave function, is missing. To repair this flaw, we experiment with a method which uses a swarm of classical trajectories to generate a ‘‘classical wave function.’’ The results are encouraging, but require substantial computer time when the number of classical variables is large. We argue that in the limit of many classical degrees of freedom, accurate calculations can be performed by using the time‐dependent Hartree method and treating some degrees of freedom by exact numerical methods (e.g., a fast Fourier transform procedure) and the others by Gaussian wave packets or any other propagation method that is accurate for a very short time. This procedure leads to a simple time domain picture of dephasing and line broadening in the case of a localized quantum system imbedded in a medium with heavy atoms.