Volume 89, Issue 12, 15 December 1988
Index of content:
Doppler‐free spectrum of the B 1Π–X 1Σ+ transition of NaK, and the perturbation and hyperfine splitting89(1988); http://dx.doi.org/10.1063/1.455334View Description Hide Description
The Doppler‐free high‐resolution spectrum of the B 1 Π–X 1Σ+ transition of NaK was measured by the technique of laser polarizationspectroscopy. The molecular constants of the B 1 Π state, which reproduced the observed 831 unperturbed line positions (v=0–6, J=1–94) with a standard deviation of 0.002 cm−1 , were determined. Many perturbed lines, which were attributed to the perturbation between the B 1 Π and c(2) 3 Σ+ states, were observed. By analyzing the energy shifts of the B 1 Π(v=4,J) levels around J=13, we estimated the rotational constantB v of the c(2) 3 Σ+ state to be 0.048 cm− 1, and the matrix element of the spin‐orbit interaction 〈c(2) 3 Σ+ v N=J J M‖H so‖ B 1Πv=4J M〉 to be 0.14 cm−1 . We found that the strongly perturbed lines split into four lines, and we identified them as a hyperfine splitting caused by a mixing of the c(2) 3 Σ+ state. The splitting into four lines is explained by the magnetic dipole interaction due to a nucleus of I=3/2.
Two‐photon excitation of dense sodium vapor near the n d 2 D 5/2, 3/2 (n=3, 4, 5) levels: Na2 1 3Σ+ g →1 3Σ+ u excimer emission89(1988); http://dx.doi.org/10.1063/1.455335View Description Hide Description
Laser excitation and ionization processes in dense (1–10 Torr) sodium vapor have been studied for laser wavelengths near the two‐photon allowed n d 2 D(n=3, 4, 5) and n s 2 S(n=4, 5, 6) states. In particular, the 1 3Σ+ g →1 3Σ+ u excimer emission in Na2, predicted in 1980 by Konowalow and Julienne and observed recently by Dinev e t a l, was studied here in greater detail. Strong excimer emission (∼830 nm) was observed for two‐photon pumping to both sides of the unresolved 4D states, and weak excimer emission was seen when pumping near the 5D levels. The excimer emission exhibits a complicated pump laser profile with a pronounced ‘‘dip’’ at the 4d 2 D two‐photon resonance. Similarly, [2+1] photonionization via the 3d 2 D and 4d 2 D states shows a dramatic decrease as the sodium density increases. These results can be attributed either to depleted 3d 2 D or 4d 2 D population due to stimulated electronic Raman scattering(SERS) or to the interference effects recently reported by Malcuit e t a l. and Krasnikov e t a l. and treated theoretically by Manykin and Afanas’ev and by Agarwal. It is argued that both mechanisms are operative. Strong ionization and SERS signals were observed at the h y b r i d resonances corresponding to 3p 2 P 3/2, 1/2 →4d 2 D transitions; however, no excimer lasing at 830 nm was detected. No excimer emission was detected upon two‐photon pumping near or at the 3d 2 D or n s 2 S(n=4, 5, 6) states. Based on these and other observations, the 1 3Σ+ g →1 3Σ+ u excimer emission is attributed to a m o l e c u l a r Raman process involving stimulated emission or six‐wave mixing via a pathway of the type 1 3Σ+ u →2hν 3Δ u →
j 3Π g →(k 3Π u , l 3Σ + u )→1 3Σ+ g →1 3Σ + u .
89(1988); http://dx.doi.org/10.1063/1.455336View Description Hide Description
The photoion yield curves of the free radicals AsH and AsH2, prepared by the reaction of H atoms with AsH3, have been measured. The adiabatic ionization potential of AsH (forming AsH+, X 2Π1 / 2) is 9.641±0.008 eV. Autoionizing Rydberg states are observed and analyzed to converge to an a 4Σ− state lying 1.94 eV higher in energy. The adiabatic ionization potential of AsH2 (forming AsH+ 2, X̃ 1 A 1) is 9.443±0.007 eV. The 3 B 1 state of AsH+ 2 is conservatively estimated from the spectrum to lie 0.60–1.46 eV higher in energy, with the lower figure expected to be close to the true value. In addition, the ion yield curves of AsH+ 3, AsH+ 2, and AsH+ from photoionization of AsH3 have been measured. From these measurements, the adiabatic ionization potential of AsH3 is 9.82±0.01 eV, the appearance potential of AsH+ 2 (+H) is 12.69±0.01 eV, and that of AsH+(+H2) is 11.295±0.05 eV. The latter two measurements, when combined with the corresponding ionization potentials, yield D 0(H2As–H)=74.9±0.2 kcal/mol and D 0(HAs–H)=66.5±0.2 kcal/mol. The value of D 0(As–H), as deduced from these measurements, depends upon an accurate heat of atomization of AsH3, which in turn requires an accurate value for ΔH ○ f 0 (As,g). An analysis of alternative values is presented, from which D 0(As–H)=64.6±0.7 kcal/mol (2.80±0.03 eV) is obtained. When these stepwise bond energies, and earlier results on PH n and NH n , are compared with the semiempirical model of Goddard and Harding, the largest discrepancy occurs for NH n . An analysis of successive ionization potentials Pn, PnH, PnH2 (Pn=N, P, As), and also Ch, ChH, ChH2 (Ch=O, S, Se) based on the same philosophy again shows a large departure from prediction for the first row elements, but fair agreement for the second and third row hydrides. The deviation of the first row hydrides from the Goddard–Harding model is attributed to the substantial ionic character of these systems.
89(1988); http://dx.doi.org/10.1063/1.455337View Description Hide Description
Emission spectra of three isotopomers of helium hydride (4HeH, 3HeH, and 3HeD) in the visible spectral region have been acquired using proton‐beam irradiation of dense helium gas (150 Torr) at 4.2 K in the presence of some solid hydrogen or deuterium. Besides the previously reported D 2Σ+→A 2Σ+ transition, near 550 nm, a second transition near 640 nm, identified as the D 2Σ+→B 2Π, has been acquired and analyzed. The spectroscopic constants for both transitions have been obtained and compared to the theoretical results based on the latest published potential curves. Further insight into the mechanism for forming HeH will be presented, which indicates that the formation process is sensitive to the hydrogen vapor pressure above the solid.
89(1988); http://dx.doi.org/10.1063/1.455338View Description Hide Description
New time and energy resolved data on vibrationally relaxed and unrelaxed emissions from the valence a 4 Π(v=0), B 2 Π(v=0,3,4,5), and the RydbergA 2 Σ+ (v=0,1,2) states of NO in Ne matrices are reported. Rydberg ↔ valence and valence ↔ valence nonradiative transitions are identified. The Rydberg → valence transitions are seen to occur after lattice relaxation accommodating the Rydberg orbital. The branching ratios for intramolecular relaxation and the measured lifetimes are described in terms of a model which combines the intramolecular spin–orbit matrix elements and Franck–Condon factors with the spectroscopically determined phonon Franck–Condon factors. For the levels B 2 Π(v=5,6), a Förster–Dexter‐type energy transfer between NO molecules is also invoked in the description of the relaxation cascade.
89(1988); http://dx.doi.org/10.1063/1.455288View Description Hide Description
New time and energy resolved spectra of the L’ 2Φ–X 2Π (0,v ″) bands of NO in Ar and Kr matrices are reported. The L’(0,v ″) bands are excited exclusively via the B’ 2Δ valence state and their lifetime is 3000±500 ns in both matrices. The quantum efficiency for L’(v=0) emission is estimated to be ≤0.04. The 2Φ–2Π transition is discussed in terms of a statically induced transition moment involving spin–orbit mixing with B’ 2Δ, but also in terms of nonadiabatic matrix elements due to the coupling to the lattice. Relaxation down to L’(v=0) is discussed in terms of matrix‐induced interstate cascading with the b 4Σ− state.
89(1988); http://dx.doi.org/10.1063/1.455289View Description Hide Description
The C–H stretch overtone spectra of methane (5–0), ethylene (5–0 and 6–0), ethane (5–0 and 6–0), propyne (4–0 and 5–0 acetylenic and 5–0 methyl C–H stretches), allene (5–0), propane (5–0 and 6–0), cyclopropane (5–0 and 6–0), dimethyl ether (5–0), and isobutane (5–0) have been recorded at temperatures between 143 and 189 K, depending on the molecule. A comparison is made to the spectra obtained at room temperature, with the goal of improved understanding of the band shapes. The temperature dependence of most of the observed bands is found to be significantly less than that expected for ‘‘simple’’ bands. For these small to medium size hydrocarbons, the temperature independence of the overtone bands is found to correlate loosely with the density of states and with the degree of saturation. Other factors are important determinants of spectral widths and temperature independence as well, such as conformational inequivalence of the C–H oscillators, and the number and positions of the oscillators. It is concluded that the vast majority of hydrocarbon C–H stretch high overtone bands have upper states which are extensively mixed with other states. This is the case even for most of the relatively small hydrocarbons. This mixing produces a broadening effect and greatly increases the transition density, thereby diluting the oscillator strength of the rovibrational transitions from that of the zero‐order approximation. The Fermi resonance type of interaction appears to be of greater importance than the Coriolis type in determining the appearance of the high overtone bands.
89(1988); http://dx.doi.org/10.1063/1.455290View Description Hide Description
Optical spectra of ten AX+ ions (A=B, Al, Ga, In; X=F, Cl, Br) have been observed in the visible and near UV; a total of 18 band systems were newly discovered. The emission was produced by chemiluminescent reactions A++X2 at low (2–10 eVCM) kinetic energy in a beam‐gas arrangement. A position‐sensitive photon countingdetector with large surface area and very low dark count rate was employed, the resolution was mostly 5–50 Å FWHM. Three types of band systems were observed: (1) For all AX+ combinations except BCl+ and BBr+, a very broad quasicontinuum with undulatory structure appears. On the basis of electronic state correlation arguments, photoelectron data, some a b i n i t i o calculations and, in one case, a known emission spectrum (InCl+) these band systems were identified as B 2Σ+–X 2Σ+ transitions. It is concluded that the excited state potentials are considerably displaced against the ground state, and their energetics are given. (2) For six species AX+, narrow band systems were observed in the 2500 Å region. They could be clearly identified as being due to C 2Π–X 2Σ+ transitions by means of comparison with the systematics of the analogous A 2Π–X 2Σ+ transitions of the isoelectronic alkaline earth halides, by the resolved fine structure, and, in the case of AlF+, by an a b i n i t i o calculation. (3) In the GaCl+, GaBr+, and InBr+spectra, narrow features accompany the C–X transitions. They are attributed to D 2Σ+–X 2Σ+ transitions, analogous to the alkaline earth halide B 2Σ+–X 2Σ+ band systems. Qualitative electronic state correlations are discussed, and the expected dominant configurations in different regions of the AX+ ground and excited states are given. These are in accord with recent a b i n i t i o results on AlF+.
Transient, collision‐induced changes in polarizability for atoms interacting with linear, centrosymmetric molecules at long range89(1988); http://dx.doi.org/10.1063/1.455291View Description Hide Description
Transient changes in polarizability during collisions between atoms and molecules give rise to interaction‐induced rototranslational Raman scattering: the scalar component of the collision‐induced polarizability Δα0 0 accounts for isotropic scattering, while the second‐rank component Δα M 2 accounts for collision‐induced depolarized scattering. We have evaluated the changes in electronic polarizability due to interactions between an atom and a molecule of D ∞h symmetry in fixed configurations, with nonoverlapping charge distributions. We have cast the resulting expressions into the symmetry‐adapted form used in spectroscopic line shape analyses. Our results are complete to order R − 6 in the atom–molecule separation R. To this order, the collision‐induced change in polarizability of an atom and a D ∞h molecule reflects not only dipole‐induced–dipole (DID) interactions, but also molecular polarization due to the nonuniformity of the local field, polarization of the atom in the field due to higher multipoles induced in the molecule, hyperpolarization of the atom by the applied field and the quadrupolar field of the molecule, and dispersion. We have analyzed the dispersion contributions to the atom–molecule polarizability within our reaction‐field model, which yields accurate integral expressions for the polarizability coefficients. For numerical work, we have also developed approximations in terms of static polarizabilities, γ hyperpolarizabilities, and dispersion energy coefficients. Estimated polarizability coefficients are tabulated for H, He, Ne, and Ar atoms interacting with H2 or N2 molecules. The mean change in polarizability Δᾱ, averaged over the orientations of the molecular axis and the vector between atomic and molecular centers, is determined by second‐order DID interactions and dispersion. For the lighter pairs, dispersion terms are larger than second‐order DID terms in Δᾱ. In both Δα0 0 and Δα M 2, first‐order DID interactions dominate at long range; other interaction effects are smaller, but detectable. At long range, the largest deviations from the first‐order DID results for Δα0 0 are
produced by dispersion terms for lighter species considered here and by second‐order DID terms for the heavier species; in Δα M 2, the largest deviations from first‐order DID results stem from the effects of field nonuniformity and higher multipole induction, for atoms interacting with N2.
The S 1–S 0(1 B 2 u –1 A g ) transition of p‐difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions89(1988); http://dx.doi.org/10.1063/1.455292View Description Hide Description
The vibronic spectroscopy of the S 1(1 B 2 u )–S 0(1 A g ) transition of p‐difluorobenzene (00 0 at 36 838 cm− 1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S 1 and S 0 frequencies, including eight previously unassigned S 1 vibrational frequencies, and the reassignment of two S 1 and one S 0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S 1–S 0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b 3g modes, notably ν2 7 and ν2 6, derive their intensity from a higher lying 1 B 1u electronic state, via vibronic coupling that is analogous to that responsible for the 1 B 2u –1 A g transition in benzene. Numerous Fermi resonances in both the S 1 and S 0 states have been identified. The prevalent Fermi resonance between ν’ 5 and 2ν6 has been analyzed with the assistance of both excitation and dispersed fluorescence spectroscopy, yielding a coupling matrix element [g 5 6 6<51‖Q 5‖50><60‖Q 6‖62>]=−1 cm− 1. Thirty‐one matrix elements describing cubic anharmonicity and involving a variety of vibrational modes have been estimated. The majority of the coupling matrix elements lie within the range ±2 cm− 1.
Multifrequency electron spin echo envelope modulation in S=1/2, I=1/2 systems: Analysis of the spectral amplitudes, line shapes, and linewidths89(1988); http://dx.doi.org/10.1063/1.455293View Description Hide Description
The nuclear modulation effect in S=1/2, I=1/2 systems is analyzed with particular emphasis on the impact of variation of the external field strength. We show that the modulation depth parameter k reaches its maximum possible value when the Zeeman and hyperfine interactions are made equal in magnitude, or ‘‘matched,’’ and that this matching plays the dominant role in determining the ESEEM (electron spin echo envelope modulation) line shapes; the distinctive ESEEM spectral features are readily understood on this basis. Through multifrequency ESEEM, it is possible to interpret the features and to utilize them to measure hyperfine interactions. We discuss several experimental strategies, involving the field dependences of ESEEM amplitudes, line shapes, and linewidths, for determining hyperfine coupling constants.
89(1988); http://dx.doi.org/10.1063/1.455294View Description Hide Description
H2S was photodissociated using an ArF excimer laser at 193 nm to form HS photofragments. LIF spectra of HS in the region of 324 nm were obtained using a pressure tuned dye laser with improved resolution of the R 1 and R Q 2 1 branches in the A 2Σ+←X 2Π3 / 2 transition. The zero field linewidths and the Zeeman splitting of the R Q 2 1 (1.5) line in a magnetic field were obtained. The latter was used to verify the g values expected for a Hund’s case (b) 2Σ+ upper state and Hund’s case (a) 2Π lower state. Depolarization of fluorescence in a magnetic field using the R Q 2 1 (3.5) line with σ‐polarized LIF excitation was used to determine an estimated lifetime of 0.15–0.3 ns for the lowest vibrational level of the 2Σ+ state. A lower limit on the lifetime of 0.17 ns was determined from the measured adsorptionlinewidths. No rotational dependence on the linewidth was observed.
Electronic properties of CuS: Experimental determination of the magnetic hyperfine interactions and permanent electric dipole moment89(1988); http://dx.doi.org/10.1063/1.455295View Description Hide Description
The technique of intermodulated fluorescence has been utilized to record the sub‐Doppler optical spectrum of gas‐phase copper monosulfide, CuS. The magnetic hyperfine interactions in the A 2Σ−(v=0) and X 2Π i (v=0) states have been analyzed and the permanent electric dipole moment for the X 2Π i state determined. The results have been compared with theoretical predictions and with those for CuO. The magnetic hyperfine parameters are significantly different from those of CuO whereas the dipole moment is nearly identical and these trends are consistent with the decrease in electronegativity of S compared to O.
89(1988); http://dx.doi.org/10.1063/1.455296View Description Hide Description
Ultrasensitive infrared laserabsorption spectroscopy in a slit supersonic expansion is used to obtain the spectrum of the HF stretching fundamental of D2HF. Both a Π←Π band due to para‐D2HF and a ∑←∑ band due to ortho‐D2HF are observed, in contrast to the H2HF spectrum which consists of the Π←Π band alone. Analysis of the spectrum indicates that the D2HF Π states are more strongly bound than the ∑ states. Doublet splittings in the Π←Π band are analyzed to determine barriers to internal rotation of D2 within the complex. The vibrationa1 predissociation rate of D2HF is approximately 25 times faster than that of H2HF, suggesting the opening of a channel which results in vibrational excitation of the D2 fragment.
89(1988); http://dx.doi.org/10.1063/1.455297View Description Hide Description
Electron paramagnetic resonance measurements are reported for Eu2 + ions occupying trigonal sites in Cs2NaYCl6single crystals. Spin–Hamiltonian parameters are reported at room temperature. A tentative model for the location of the impurity in the lattice is discussed.
The photoelectron spectrum of HCl and DCl studied with ultraviolet excitation, high resolution x‐ray excitation, and synchrotron radiation excitation: Isotope effects on line profiles89(1988); http://dx.doi.org/10.1063/1.455298View Description Hide Description
The HCl and DCl molecules have been studied with monochromatized x‐ray, ultraviolet, and synchrotron radiation excited photoelectron spectroscopy.Isotope effects are detected in the outer and inner valence bands using all the different excitation sources. These effects are used to describe the nature of the potential curves for the 5σ− 1 state and various inner valence correlation states. The 5σ− 1 state is shown to predissociate at 18.0 eV into Cl+(3 P)+H(2 S). The potential curves for the two outermost states in the 4σ− 1 region are approximately determined. The outermost state is shown to be repulsive, whereas the second state, that corresponds mostly to the 4σ− 1 single hole state, is found to be bound. One weak structure that must be associated with the 2∏ manifold of states is observed at 28.6 eV binding energy. Three structures at binding energies larger than 40 eV are reported for the first time.
89(1988); http://dx.doi.org/10.1063/1.455299View Description Hide Description
Rotation–vibration interactions between the two lowest frequency normal modes of H2CO, the out‐of‐plane bend and the in‐plane wag, are studied using classical trajectories. The dynamics is investigated for a range of rotational angular momenta, J, and energy values. Vibrational energy flow is elucidated by examining trajectories in several different canonical representations. The a‐axis Coriolis term, which is quadratic in the normal coordinates, accounts for most of the coupling, as seen by comparing plots in the normal mode representation and one in which the Coriolis term has been subsumed into the zero‐order Hamiltonian. In the former, the modes are more strongly coupled as the projection of J onto the body‐fixed z axis increases; in contrast, the Coriolis adapted normal modes are more decoupled. Making use of the observed decoupling, the rovibrational Hamiltonian is reduced to an effective one degree‐of‐freedom rotational Hamiltonian whose dynamics depends on the vibrational excitation. Model spectra have been obtained using the semiclassical method of Gaussian wave packet propagation of Heller [J. Chem. Phys. 6 2, 1544 (1975)]. Semiclassical and full quantum results analogous to the observed classical dynamics are presented.
89(1988); http://dx.doi.org/10.1063/1.455300View Description Hide Description
Energy disposal to the CO product formed upon the 351 nm photodissociation of W(CO)6 has been monitored using the method of time‐resolved infrared laserabsorption spectroscopy. The nascent CO product can be characterized by effective vibrational, rotational, and translational temperatures; T v =1080±60 K, T 0 r (v=0)=560±50 K, and T 0 t (v=0 J=10) =1550±200 K. These results are considered in light of various models for energy disposal in the photofragmentation reaction. Vibrational energy disposal is consistent with a modified version of phase space theory termed ‘‘early’’ phase space theory, EPST. Rotational and translational energy release is not consistent with phase space theory or its variants, e.g., EPST and the separate statistical ensemblesmodel, but appears in qualitative accord with an impulsive model. We propose that, in general, vibrational energy release occurs early in the exit channel for the reaction, relative to rotational and translational energy release.
Negative ion formation in K(n d)–CS2 collisions: Detection of electric‐field‐induced detachment from CS− 289(1988); http://dx.doi.org/10.1063/1.455301View Description Hide Description
Charged particle production in thermal‐energy K(n d)–CS2 collisions is investigated for intermediate values of n. The data show that collisions result in the formation of relatively long‐lived CS− 2 ions and of free electrons. A fraction of the CS− 2 ions is observed to undergo rapid electric‐field‐induced detachment in fields of only a few kilovolts per centimeter and this novel phenomenon is discussed.
Electron degradation and yields on initial products. II. Subexcitation electrons in molecular nitrogen89(1988); http://dx.doi.org/10.1063/1.455302View Description Hide Description
Subexcitation electrons lose their kinetic energy through vibrational excitation, rotational excitation, and elastic collisions in molecular gases. Initial yields of vibrationally and rotationally excited states of nitrogen molecules are calculated by using the Spencer–Fano equation (SFE) and its simplification, the continuous‐slowing‐down approximation (CSDA), both in time‐independent and time‐dependent representations. One focus of the present study is a close comparison of the CSDA with the rigorous treatment of the SFE in the subexcitation domain. The present result reveals for the first time distinct energy regions in which either vibrational excitation or rotational excitation dominates. This recognition explains the different time dependence of the yields of vibrational and rotational excitation.