Volume 84, Issue 5, 01 March 1986
Index of content:
84(1986); http://dx.doi.org/10.1063/1.450362View Description Hide Description
The molecular response in laser intracavity experiments where molecules interact simultaneously with a radio frequency field and an infrared one is analyzedtheoretically. In the quantized field approach the molecule–radio frequency interaction is treated exactly. For the laser intensity the density matrix equations are solved either in a third‐order perturbation treatment or through a matrix continued fraction analysis. The resonance line shapes in different two‐level and three‐level configurations are analyzed numerically.
84(1986); http://dx.doi.org/10.1063/1.450363View Description Hide Description
PolarizationCARSspectroscopy is known to considerably enhance the CARS sensitivity of weak Raman resonances by background suppression in transparent media. Experimental control of the spectralline shape through polarization enables the determination of molecular parameters by line shapeanalysis. We demonstrate that polarizationCARS can also be applied to absorbing substances, i.e., under electronic resonance conditions, for the case of pseudoisocyanine chloride. The spectra, recorded in the multiplex mode, are interpreted theoretically and the parameters involved in the third order susceptibility are determined from experiment. Multiplex performance of polarizationCARSspectroscopy is also demonstrated for solutions of pyridine in water and benzene in carbon tetrachloride as transparent systems.
The 2 A 1–2 E electronic spectrum of methoxy, CH3O: Vibrational progressions, and spin‐orbit and Jahn–Teller couplings84(1986); http://dx.doi.org/10.1063/1.450364View Description Hide Description
The 310 nm electronic emission system of jet‐cooled methoxy radical has been examined at 4 cm− 1 resolution. The spectrum shows extensive progressions in the upper and lower state CO stretch frequencies, v ’ 3 =660±2 cm− 1 and v ‘ 3 =1045±3 cm− 1. Two other lower state frequencies have been determined: the hydrogen stretch v ‘ 1 =2749±5 cm− 1 and the hydrogen scissors v ‘ 5 =1499±6 cm− 1. The Jahn‐Teller effect activates v ‘ 5 with a coupling constant of k 2=0.28±0.06, but does not appreciably activate either v ‘ 4 or v ‘ 6. The corresponding frequencies have been observed in deuterated methoxy. The change in the spin‐orbit splitting upon deuteration, dropping from 62 to 56 cm− 1, indicates that the purely electronic spin‐orbit splitting is 98±11 cm− 1, while the vibronic reduction factor in the hydride is 0.63±0.06.
84(1986); http://dx.doi.org/10.1063/1.450365View Description Hide Description
Measurements of absorption in the fundamental (4000 to 5000 cm− 1) vibrational band of H2 have revealed new features in the collision‐induced spectrum which attain their maximum intensity under near‐critical conditions (T c =32.98 K for p a r a‐H2). The new features are relatively sharp ‘‘spikes’’ (FWHM≊5 cm− 1) which occur at the frequencies of the Q 1(0) and/or Q 1(1) transitions, and the effect is most prominent in pure p a r a‐H2. The behavior of these features as a function of temperature and of o r t h o‐/p a r a‐H2 ratio has been investigated, they have also been detected in the spectrum of D2 near the critical point. An interpretation is proposed on the basis that long‐lived spatial fluctuations in density give rise to a spoiling of the normally active mechanism of destructive intercollisional interference.
84(1986); http://dx.doi.org/10.1063/1.450366View Description Hide Description
The rotational spectra of NH3–CO, ND3–CO, ND2H–CO, NDH2–CO, NH3–1 3CO, and NH3–N2 have been measured by molecular beam electric resonance. The K=0 ground vibrational state transitions for these species were fit to a linear molecule Hamiltonian and the following constants were obtained for NH3–CO; (B+C)/2 (MHz)=3485.757(2), D J (kHz)=110.2(2), e Q q N a a (MHz)=−1.890(7), μ a (D)=1.2477(8). These constants were also determined for
ND3–CO [3078.440(7), 75.7(8), −2.028(15), 1.2845(9)], NHD2–CO [3202.303(4), 86.8(6), −1.972(11), 1.2686(8)], NH2D–CO [3338.235(4), 98.9(6), −1.916(12), 1.2546(8)], NH3–1 3CO [3451.684(5), 108.7(7), −1.870(15), 1.2452(8)]. For NH3–N2 (B+C)/2=3385.76(21), D J =117.(10), and μ a =1.069(14). For NH3–CO three ‖ΔJ‖=1, K=0 progressions were seen along with two ‖ΔJ‖=1, K=1 progressions, suggesting nonrigidity in the complex. The internal rotation of the NH3 subunit about its C 3 axis is expected to be essentially free, but this motion, by itself, is not sufficient to explain
the observed spectra, thus, large amplitude dynamics are occurring in at least two degrees of freedom. The quadrupole coupling constants, e Q q N a a indicate that in each of the isotopes of NH3–CO the NH3 subunit has its C 3 axis relatively rigidly oriented at an angle of approximately 36° with respect to the line connecting the centers of mass of the two subunits. The structure is not hydrogen bonded; the N atom is closest to the CO subunit. The orientation of the CO subunit is not established. The distance between the N atom and the center of mass of the CO unit (R N–CO) is 3.54(3) Å. The spectroscopic constants suggest that the weak bond stretching force constant is quite small (0.01 mdyn/Å) but compatible with the long bond length.
84(1986); http://dx.doi.org/10.1063/1.450367View Description Hide Description
The rotationally cold spectrum of the B̃ 2Π–X̃ 2Π system of ClCN+ has been obtained by the crossed free jet‐electron beam method. The narrowing of vibrational bands has allowed the assignment of many of the major features in the spectrum. The (Cl–C) stretching vibrational frequencies have been determined to be 823 and 526 cm− 1 for the ground and excited electronic states, respectively, and the (C 3/4 N) stretching vibrational frequency for the ground electronic state has been determined to be 1915 cm− 1. The difference in spin‐orbit coupling constants for these two electronic states was also measured in the spectrum to be 87 cm− 1. The difference in vibrational frequencies and the difference in spin‐orbit coupling constants for these electronic states, in comparison with similar values for the isoelectronic molecules ClC2H+ and SCN, allow a fairly complete picture of the π bonding in these molecules to be formed.
84(1986); http://dx.doi.org/10.1063/1.450368View Description Hide Description
Far IR rotational transitions between the four lowest rotational levels in the X 3Σ− vibronic ground states of OH+ and OD+ have been observed by laser magnetic resonancespectroscopy.Ground state molecular constants, including the three g factors, have been determined and employed in the calculation of a Born–Oppenheimer equilibrium geometry. The centrifugal distortion of the spin‐rotation interaction is found to have a significant effect on the determination of other molecular constants. Hyperfine splittings have been resolved and analyzed for OH+, but could not be observed in OD+spectra with a 6 MHz collision‐broadened linewidth.
84(1986); http://dx.doi.org/10.1063/1.450369View Description Hide Description
Resonance Raman (RR) profiles of the 1005, 1155, and 1525 cm− 1 modes of β‐carotene dissolved in carbon disulfide have been measured at room temperature and at 172 K. Previous studies, based upon room temperature measurements, have indicated that inhomogeneous (i.e., site) broadening may be important for this system. Our measurements are the first RR data for this system at two temperatures. Such data are necessary in order to study the relative importance of inhomogeneous broadening and thermal broadening. Using previously developed transform techniques, we analyze our RR data by calculating profile line shapes directly from our measuredoptical absorption data for each temperature. The assumptions underlying this analysis do not include inhomogeneous broadening, and the calculations yield profile line shapes which are in quite good overall agreement with the measured profile line shapes for all three modes at both temperatures. We have also extended the transform calculations in order to incorporate inhomogeneous broadening. However, the agreement between the measured and calculated RR profile line shapes is not substantially improved by the inclusion of inhomogeneous broadening in the transform analysis.
84(1986); http://dx.doi.org/10.1063/1.450837View Description Hide Description
A model for pressure narrowing in collision‐induced absorption is developed, using transition probabilities given by the Ornstein–Uhlenbeck process, with Gaussian induced dipole moments. The autocorrelation function for this model can be obtained analytically, and its principal properties readily described. The resultant line shape is a three‐parameter function, which exhibits both peak narrowing and base broadening. The transition to the narrowed line shape is found to be gradual. It is found that if angular modulation is ignored, the resultant line shape has a cusp about zero frequency, so that angular modulation contributes not only quantitatively, to the width of the line, but qualitatively, to the line shape. These results are model independent. By including angular modulation, the line shape obtained is also descriptive of the overlap‐induced component of the Q branch. The last comprises a generalization of the Levine–Birnbaum line shape to include pressure narrowing.
84(1986); http://dx.doi.org/10.1063/1.450370View Description Hide Description
A detailed X‐band EPR study of a single crystal of CaCd (CH3COO)4⋅6H2O doped with Cu2 + has been made from room temperature to liquid helium temperature. The crystal undergoes only one reversible structuralphase transition over this temperature range; this occurs at 130±1 K. As the temperature is lowered, in the general directions of the magnetic field, the single set of four hyperfine lines at room temperature splits into four sets of four hyperfine lines each at just below 130±1 K, indicating the presence of four magnetically inequivalent ions below this temperature. Careful EPR measurements below the phase‐transition temperature for the magnetic field directions in several selected planes have been made to study the orientations of the magnetic axes of the four inequivalent Cu2 + ions. It is found that at lower temperatures the magnetic axes of the four inequivalent ions are symmetrically located, 5° away from the c axis, retaining the fourfold symmetry relating the chains. The principal values and direction cosines of the g 2 and A 2 tensors at different temperatures, and for different Cu2 + sites, are evaluated rigorously on a digital computer, using the method of least‐square fitting, employing simultaneously all resonant line positions obtained for several orientations of the external magnetic field in three mutually perpendicular planes.
84(1986); http://dx.doi.org/10.1063/1.450371View Description Hide Description
The 488 nm photoelectron spectra of HCO− and DCO− show vibrational structure in the X̃ 2 A’ state of neutral formyl radical up to 10 000 cm− 1 above the vibrational ground state.Electron affinities are found to be 0.313±0.005 eV for HCO and 0.301±0.005 eV for DCO. The CH bond strength and heat of formation of HCO− and the gas phase acidity of formaldehyde are derived from these data. A Franck–Condon analysis of the photoelectron spectra provides an estimate of the equilibrium geometry of the anion. Transitions to excited vibrational states of HCO enable the determination of a complete set of quadratic anharmonicities.
84(1986); http://dx.doi.org/10.1063/1.450372View Description Hide Description
A dye laser pumped Br2 B 3Π(0+ u ) → X 1Σ+ g laser has been studied. Spectroscopic assignments have shown that lasing occurs from 10≤J’≤63 in 12≤v’≤17 using Rhodamine 590 dye. The output appeared limited to the 79–81 isotope of Br2. By utilizing stimulated emission as a monitor for laser excitation spectra, dramatic increases in the resolution were observed that exceeded the normal resolution of the dye laser. The Br2 laser operated at Br2 pressures of up to 60 Torr. A simple model to explain the characteristics of the Br2 laser is described.
84(1986); http://dx.doi.org/10.1063/1.450323View Description Hide Description
The electronic spectrum of the amino acid tryptophan has been measured in the environment of a cold, supersonic molecular beam. The resonantly enhanced two‐photon ionizationspectrum of tryptophan shows some features characteristic of more volatile indole derivitives, however the region of the spectrum near the origin shows distinctive low frequency structure absent from the simpler indole containing molecules. The power dependence of the spectrum reveals features which can be attributed to several conformers of tryptophan in the molecular beam. One of these conformers shows a nearly harmonic 26 cm− 1 vibrational progression which does not appear in the spectra of other indole derivitives, and the intensity pattern of this progression indicates that this particular conformer undergoes a significant geometry change upon electronic excitation. The lack of many extensive vibrational progressions in the electronic spectrum indicates that the excited state conformers of tryptophan are similar to those in the ground electronic state. The identification of a small number of stable tryptophan conformers is important for understanding the photophysics of tryptophan in solution.
On the nature of the two low‐lying singlet states of anthracene: Vibronic activity in the two‐photon excitation spectrum84(1986); http://dx.doi.org/10.1063/1.450324View Description Hide Description
The two‐photon excitation spectrum of anthracene in a n‐heptane matrix at 10 K has been measured in the energy region 26 000–32 000 cm− 1. Experimental evidence of two‐photon band assignment to vibronically induced B 1u ×b 1u and B 2u ×b 2u transitions is given. In particular, the two‐photon spectrum above ≂28 000 cm− 1 shows several vibronic origins built on b 2u vibrations and progressions of a g modes on them. The lowest ππ* absorption region (<28 000 cm− 1) has, on the contrary, negligible intensity and very weak B 1u ×b 1u bands are observed. These data can be rationalized in terms of vibronic coupling between electronic states induced by normal
coordinates of b 1u and b 2u symmetry. By means of the Herzberg–Teller theory and displacing the molecule away from equilibrium along the normal coordinate we have calculated the vibronic interaction between electronic states in the orbital following approach. The results based on CNDO/S‐CI wave functions show that B 2u ×b 2u transitions have a larger vibronic activity than the B 1u ×b 1u transitions. The 1132, 1384, and 1393 cm− 1 b 2u modes are particularly strong in inducing two‐photon intensity through a vibronic coupling mechanism involving essentially the ground and the final 1 1 B 2u state. The B 2u ×b 2u two‐photon amplitude tensors are not sensitive to the method of calculation. B 1u ×b 1u tensors have instead a more pronounced dependence on the method used. This is due to the fact that in the two‐photon sums most of the intermediate states play an equivalent
role in determining the amplitude tensor, in contrast with the B 2u ×b 2u case. It is important to use a reasonably correct representation of the excited statewave functions. It is shown that more accurate calculations (INDO/S and CNDO/S with increased CI) lead to better agreement of the total vibronic intensity of the 1 1 B 1u state with experiment and predict the largest activity for the 648 cm− 1 b 1u mode, as observed in the spectrum.
84(1986); http://dx.doi.org/10.1063/1.450325View Description Hide Description
An examination of several composite pulse quadrupole echo sequences designed for the broadband excitation of I=1 systems shows that these sequences reduce but do not eliminate finite pulse width effects. It is demonstrated both experimentally and theoretically that these sequences can also introduce additional distortions. The origin of these distortions and several aspects of the evaluation of broadband excitation sequences are discussed. It is shown that these distortions can be eliminated by employing higher order composite pulses. Finally the viability of composite pulse excitation for systems undergoing chemical exchange is discussed.
Depolarized differential Rayleigh light scattering (DDRLS) by uncorrelated molecules with degenerate electronic states84(1986); http://dx.doi.org/10.1063/1.450326View Description Hide Description
A detailed analysis of DDRLS is performed for arbitrary scattering angle θ and all molecular symmetries. Different quantum mechanisms are shown to be responsible for the effect at θ=π/2 and θ≠π/2. For one magnetic point group, the DDRLS is described by ctensors only (i.e., by tensors asymmetric with respect to time inversion), whereas for 13 other magnetic point groups—by itensors (symmetric with respect to time inversion); in the remaining magnetic point groups it is dependent on itensors as well as ctensors, or does not exist altogether. Equations are derived permitting, by way of DDRLS measurements, the determination of the tensor components G and A for seven magnetic point groups. For molecules with an isotropic electromagneticpolarizabilitytensorG a simple relation between the DDRLS and the angle of rotation of the light polarization plane in the effect of natural optical activity is established.
Infrared diode laser kinetic spectroscopy of the CCO radical in the X̃ 3Σ− state generated by the excimer laser photolysis of carbon suboxide84(1986); http://dx.doi.org/10.1063/1.450835View Description Hide Description
The ν1 band of the CCO radical in the X̃ 3Σ−ground electronic state has been observed in the gas phase by diode laser kinetic spectroscopy. The CCO radical was generated by the 193 or 248 nm excimer laserphotolysis of carbon suboxide. By fixing ground state parameters to the microwave values, the band origin and the vibrational changes of the rotational (α B =B 0−B 1) and spin–spin interaction (αλ=λ1−λ0) constants have been determined to be 1970.864 34(95), 0.003 075 4(85), and 0.008 3(12) in cm− 1 with 2.5 standard errors in parentheses.
Site‐selective photochemistry and vibrational analysis of matrix‐isolated 1,3,5,7‐octatetraene radical cations in different conformations84(1986); http://dx.doi.org/10.1063/1.450327View Description Hide Description
Upon ionization by x irradiation, a l l‐t r a n s 1,3,5,7‐octatetraene (OT) gives an electronic absorption (EA) spectrum which indicates the formation of at least five different (peri)planar conformations (‘‘rotamers’’) of OT+̣ next to the parent cation. Some of these undergo specific photochemical interconversions which are discussed on the basis of a complete scheme of OT+̣ rotamers and their connections via single bond‐rotation processes. None of these photoreactions lead to any other than the initially observed six species which seem to form a distinguished set within the 20 possible OT+̣ rotamers. By very narrow bandwidth irradiation, interconversions can be induced to take place in a site‐selective fashion. The resulting well‐resolved difference spectra allow an analysis of the site dependence of the different ion’s first two EA bands. Surprisingly, the extent of this site dependence varies greatly between different OT+̣ rotamers and between different electronic transitions of a given rotamer. Finally, a detailed vibrational analysis of the first two absorption bands of a l l‐t r a n s OT+̣ is given.
84(1986); http://dx.doi.org/10.1063/1.450328View Description Hide Description
A diffusion‐reaction model with time‐dependent reactivity is formulated and applied to unimolecular reactions. The model is solved exactly numerically and approximately analytically for the unreacted fraction as a function of time. It is shown that the approximate analytical solution is valid even when the system is far from equilibrium, and when the reactivity probability is more complicated than a square‐wave function of time. A discussion is also given of an approach to problems of this type using a stochastically fluctuating reactivity, and the first‐passage time for a particular example is derived.
Semiclassical calculation of cross sections for vibration–rotation energy transfer in HF–HF collisions84(1986); http://dx.doi.org/10.1063/1.450329View Description Hide Description
Cross sections for HF(10)+HF(00) → HF(v 1 j 1)+HF(v 2 j 2) are calculated using a semiclassical method in which the relative translational motion is treated classically, whereas the vibrational and rotational motion of both molecules are quantized. In order to decouple the rotation projection states we introduce the large j approximation in the coupling elements. The results of 98‐quantum state calculations at three energies are reported and qualitatively compared with recent laser experiments.