Volume 105, Issue 14, 08 October 1996
Index of content:

Theory of frequency‐shifted excitation by phase‐incremented pulses in nuclear magnetic resonance
View Description Hide DescriptionFrequency‐shifted excitation by phase‐incremented pulses of arbitrary shape can be solved by introducing a second rotating frame. The results show that a phase‐incremented pulse can be decomposed into an infinite number of unsymmetrically amplitude‐scaled and phase‐shifted effective radio frequency (rf) fields, which are responsible for the unsymmetrical and phase‐shifted centerband and sideband excitations. In addition to a universal phase shift associated with each band of excitation, a phase inversion occurs when the scaling factor of the effective rf field becomes negative. Also if the total phase increment of the pulse is not equal to 2kπ (k integer) an additional phase shift equal to the total phase increment will be introduced. By properly choosing the total phase increment the phase shift of the centerband can be compensated. A computer program based on the Bloch equations is developed to calculate directly the excitation profiles of phase‐incremented pulses, which agrees well with the theoretical predications.

Study of electronically excited states of ozone by electron‐energy‐loss spectroscopy
View Description Hide DescriptionElectronically excited states of ozone have been studied by electron‐energy‐loss spectroscopy. Two broad bands without visible vibrational structure are observed at 1.8±0.2 eV and 2.05±0.05 eV under scattering conditions favoring singlet excitation, that is a scattering angle of ϑ=10° and residual energyE _{ r }=20 eV. The lower is assigned as ^{1} A _{2}, the higher as ^{1} B _{1} (Chappuis band). Bands with rich vibrational structure are observed under scattering conditions favoring triplet excitation, ϑ=30°–135° and residual energyE _{ r }=1–3 eV. At least two vibrational progressions can be discerned. The first has an origin at 1.30 eV, the origin of the second cannot be determined unambiguously, it is either at 1.53 or 1.45 eV. The well‐known Hartley band and a number of other singlet and triplet excited states are observed at higher energy losses. Excitation functions and angular distributions of the triplet band at 1.30 eV and of the Hartley band are presented. The absolute value of the differential cross section for excitation of the Hartley band is given.

Electron spin resonance rare gas matrix studies of ^{12}CO_{2} ^{−}, ^{13}CO_{2} ^{−}, and C^{17}O^{−} _{2}: Comparison with ab initio calculations
View Description Hide DescriptionThe ^{12}C^{16}O_{2} ^{−}, ^{13}C^{16}O_{2} ^{−}, ^{12}C^{17}O_{2} ^{−}, and ^{12}C^{16,17}O_{2} ^{−} radical anions have been generated by four independent methods and isolated in neon and argon matrices for detailed ESR(electron spin resonance) investigations. Included with these experimental measurements for the various magnetic parameters of CO^{−} _{2} are high level ab initio calculations (MR SD‐CI and others) of the ^{13}C and ^{17}O hyperfineAtensors. Some of the calculations included the effects of a 42‐atom neon cage on the electronic structure of CO^{−} _{2}. Previous ESR studies of CO^{−} _{2} have been conducted in more perturbing environments, such as ionic crystals, where the close proximity of the counter cation can alter the anion’s properties. A comparison of the earlier measurements in more interactive materials with these theoretical and rare gas matrix results reveals a significantly different distribution of the spin density. The neon magnetic parameters (MHz) for CO^{−} _{2} are g _{ x }=2.0018, g _{ y }=1.9964, g _{ z }=2.0010; for ^{13}C, A _{ x }=320.4, A _{ y }=296.1, A _{ z }=394.5; for ^{17}O, A _{ x }=−81.6, A _{ y }=−74.9 and A _{ z }=−151.8. The argon results are similar to these neon values; isotropic spectra in argon were also observed at elevated temperatures that yielded g _{iso} and A _{iso} parameters consistent with the low temperature (4 K) anisotropic spectra.

Electronic spectroscopy of jet‐cooled benzylidenecyclobutane, a sterically hindered styrene
View Description Hide DescriptionThe electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two‐photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited statepotential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground statesurface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°–26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule.

Magnetic field effects on the single‐rovibronic‐level fluorescence of S _{1}(^{1} B _{1}) pyrimidine: Study of the singlet–triplet coupling by level anticrossings and quantum beats
View Description Hide DescriptionLevel anticrossings and quantum beats have been observed for the fluorescence from single rotational levels of the vibrationless S _{1} pyrimidine in a supersonic free jet. The density of triplet levels (ρ) and the coupling matrix elements (v) deduced from the data obtained for R(0) transition confirm that the singlet–triplet coupling belongs to the weak coupling limit (vρ≪1) of interstate interaction, in which electronic relaxation from the prepared singlet state to the triplet manifold does not occur in the absence of collisions.

Dynamics of high‐n Rydberg states employed in zero kinetic energy‐pulsed field ionization spectroscopy via the F ^{1}Δ_{2}, D ^{1}Π_{1}, and f ^{3}Δ_{2} Rydberg states of HCl
View Description Hide DescriptionThe intensity anomalies in the spin–orbit and rotational branching ratios in the zero kinetic energy pulsed‐field ionization (ZEKE‐PFI) spectra via the F ^{1}Δ_{2}, D ^{1}Π_{1}, and f ^{3}Δ_{2}Rydberg states of HCl have been studied. In general, the branching ratios are observed to depend on three parameters employed in the pulsed field ionization experiment: (i) the delay time between excitation and ionization; (ii) the magnitude of the bias electric field; and (iii) the magnitude of the applied pulsed electric field. The results can be rationalized on the basis of the increasing number of autoionization decay channels that become available to the high‐nRydberg states as each ionization threshold is surpassed. The delay dependence of the ZEKE‐PFI spectra via the F ^{1}Δ_{2} state has been analyzed in more detail by ab initio calculations. These calculations show that the observed spin–orbit branching ratios can be reproduced thereby giving evidence for a nonexponential decay of the high‐nRydberg states (n≊100).

Fully complex implementation of the Robert–Bonamy formalism: Half widths and line shifts of H_{2}O broadened by N_{2}
View Description Hide DescriptionThe complex semiclassical formalism of Robert and Bonamy is used to calculate both half widths and line shifts for water vapor in a bath of nitrogen. The assumed intermolecular potential is a combination of electrostatic, Lennard‐Jones 6‐12 atom–atom, induction, and dispersion terms. The complex valued resonance functions have been previously evaluated when the assumed potential was electrostatic only. In this work these functions are evaluated when the potential is extended to include the atom–atom terms. Calculations made in the 3ν_{1}+ν_{3} vibrational band of H_{2}O are in good agreement with experimental results for both the half width and line shifts. It is shown that the imaginary parts are important for both the line shift and half width calculations.

Vibronic analysis of the B̃ ^{2} A′–X̃ ^{2} A″ laser‐induced fluorescence of jet‐cooled C_{2}H_{5}S
View Description Hide DescriptionEthylthio (C_{2}H_{5}S) radicals were formed on laser photolysis at 248 nm of diethyl disulfide (C_{2}H_{5}SSC_{2}H_{5}) or ethyl mercaptan (C_{2}H_{5}SH) in a free‐jet expansion. The fluorescence excitation spectrum was recorded in the spectral region 398–432 nm. The origin lies at 23 519.6 cm^{−1}, approximately 799 cm^{−1} greater than previously reported. Two main progressions with spacings near 420.5 cm^{−1} (C–S stretch) and 256.0 cm^{−1} (CCS bend) are dominant. Additional active fundamental vibrational modes of the B̃ state are at 718.4, 862.8, 1054.6, 1158.9, and 1203.3 cm^{−1}. Observation of hot bands enables accurate determination of four low‐lying vibrational modes of the ground state at 271.9, 296.0, 478.3, and 672.4 cm^{−1}. The dispersed fluorescence was recorded in the spectral region 415–525 nm. We identified several additional vibrational modes of the X̃ state at 890, 957, 1075, 1257, 1290, 1470, 2950, and 3050 cm^{−1}. Theoretical calculations at the MP2 level were performed to predict vibrational frequencies of both B̃ and X̃ states, and for the latter state were also with the B3‐LYP density functional theory; the results agree satisfactorily with experimental observations.

Spectroscopic constants and potential energy surfaces for silanone (H_{2}SiO), hydroxysilylene (HSiOH), the hydroxysilylene dimer, and the disilynyl radical (Si_{2}H)
View Description Hide DescriptionAb initio quantum mechanical methods were employed to study the spectroscopic constants and potential energy surfaces of H_{2}SiO, HSiOH, the HSiOH dimer, and the Si_{2}H radical. Consideration of the spectroscopic constants of silanone, cis‐ and trans‐HSiOH and Si_{2}H began with the TZ2P SCF level of theory. We predict a strongly bonded cis‐HSiOH dimer. The structure of the cis‐HSiOH dimer was optimized at the DZP SCF, DZP CISD, DZP+diff CISD and DZP MP2 levels. The hydrogen bondenergy of the dimer is 14.8 kcal/mol at the DZP MP2 level and 12.0 kcal/mol at the DZP CCSD/DZP CISD level. The vibrational frequency of one Si–O bond stretch in the HSiOH dimer is 967 cm^{−1} at the DZP MP2 level, close to the 951 cm^{−1} and 986 cm^{−1} fundamentals observed experimentally for H_{ x }Si_{ y }O_{ z } aggregates. Therefore, it is possible that the HSiOH dimer has been observed in matrices. The potential energy surface of the Si_{2}H radical was studied initially at the DZP CISD level. We found a bent C _{ s } ^{2} A″ Si_{2}H structure which is 10.8 kcal/mol higher in energy than the C _{2v } ^{2} B _{1} structure. The C _{2v } Si_{2}H structures were optimized at the TZ2P (f,d) CCSD level. The ^{2} B _{1} state is predicted to lie ΔE _{0}=1.6 kcal/mol lower in energy than the ^{2} A _{1} state of Si_{2}H radical.

Decoupling of lithium and proton self‐diffusion in supercooled LiCl:7H_{2}O: A nuclear magnetic resonance study using ultrahigh magnetic field gradients
View Description Hide DescriptionSelf‐diffusion coefficients of lithium ions and water protons (D _{Li} and D _{H}) in the glass‐forming electrolyte LiCl:7H_{2}O have been measured by nuclear magnetic resonance spin‐echo experiments using ultrahigh static magnetic field gradients up to 184 T m^{−1}. The measurements were complemented by measurements of ^{7}Li and ^{1}H spin‐lattice relaxation times. The data cover the temperature range from 313 K down to 173 K, i.e., 34 K above the glass transition temperature T _{ g }=139 K. In this range D _{Li} and D _{H} change over five orders of magnitude. The self‐diffusion data exhibit a strong non‐Arrhenius temperature dependence which is typical for fragile glass formers. In the supercooled regime the ratio of the self‐diffusion coefficients D _{H}/D _{Li} increases gradually with decreasing temperature, reflecting a decoupling of these diffusive modes. These results are discussed in relation to the behavior of the viscosity, electrical conductance and reorientational correlation time of water in this temperature range. It is found that lithium ion diffusion is closely coupled to these other transport processes, while proton diffusion begins to decouple at T<1.5 T _{ g }. Additionally, an analysis of ^{1}H and ^{7}Li magnetic relaxation rates 1/T _{1} is given. It is found that the intermolecular modes causing ^{1}H–^{1}H dipolar relaxation and ^{7}Li quadrupolar relaxation also decouple from the viscosity. The results are discussed in the framework of similar phenomena observed with other fragile glasses and, more specifically, of structural changes known to occur in supercooled LiCl:H_{2}O systems.

Sub‐Doppler Zeeman spectroscopy of pyrazine: S _{1} ^{1} B _{3u }–S _{0} ^{1} A _{ g } 0^{0} _{0} band
View Description Hide DescriptionHigh‐resolution fluorescence excitation spectrum of the S _{1} ^{1} B _{3u }−S _{0} ^{1} A _{ g } 0^{0} _{0} band of pyrazine in a collimated molecular beam and the change with the external magnetic field were measured. Transitions to each molecular eigenstate were resolved. For the J ^{′}=0 level, the magnetic fieldeffect was not very large. By taking account of the mixing of fine‐structure levels, we analyzed and simulated the spectra of the P(1) line. The fairly good agreements with the observed results are obtained. The spectral lines of the J ^{′}≠0 levels were observed to split remarkably and the intensity decreased drastically with the magnetic field even at smaller than 100 Gauss. The spectra at zero field and in the magnetic field have been analyzed by taking account of the spin‐orbit interaction between the singlet and triplet vibrational levels and Zeeman interaction in the triplet manifold. The intersystem crossing and the magnetic fieldeffect in the S _{1} origin level of pyrazine are studied as a function of the number of coupled triplet levels.

Studies of spin relaxation and molecular dynamics in liquid crystals by two‐dimensional Fourier transform electron spin resonance. I. Cholestane in butoxy benzylidene‐octylaniline and dynamic cage effects
View Description Hide DescriptionTwo‐dimensional Fourier transform (2D‐FT) electron spin resonance(ESR) studies on the rigid rodlike cholestane (CSL) spin‐label in the liquid crystalsolvent 4O,8 (butoxy benzylidene octylaniline) are reported. These experiments were performed over a wide temperature range: 96 °C to 25 °C covering the isotropic (I), nematic (N), smectic A (S _{ A }), smectic B (S _{ B }), and crystal (C) phases. It is shown that 2D‐FT‐ESR, especially in the form of 2D‐ELDOR (two‐dimensional electron–electron double resonance) provides greatly enhanced sensitivity to rotational dynamics than previous cw‐ESR studies on this and related systems. This sensitivity is enhanced by obtaining a series of 2D‐ELDOR spectra as a function of mixing time, T _{ m }, yielding essentially a three‐dimensional experiment. Advantage is taken of this sensitivity to study the applicability of the model of a slowly relaxing local structure (SRLS). In this model, a dynamic cage of solvent molecules, which relaxes on a slower time scale than the CSL solute, provides a local orienting potential in addition to that of the macroscopic aligning potential in the liquid crystalline phase. The theory of Polimeno and Freed for SRLS in the ESR slow motional regime is extended by utilizing the theory of Lee et al. to include 2D‐FT‐ESR experiments, and it serves as the basis for the analysis of the 2D‐ELDOR experiments. It is shown that the SRLS model leads to significantly improved non‐linear least squares fits to experiment over those obtained with the standard model of Brownian reorientation in a macroscopic aligning potential. This is most evident for the S _{ A } phase, and the use of the SRLS model also removes the necessity of fitting with the unreasonably large CSL rotational asymmetries in the smectic phases that are required in both the cw‐ESR and 2D‐ELDOR fits with the standard model. The cage potential is found to vary from about k _{ BT } in the isotropic phase to greater than 2k _{ BT } in the N and S _{ A } phases, with an abrupt drop to about 0.2k _{ BT } in the S _{ B } and C phases. Concomitant with this drop at the S _{ A }–S _{ B } transition is an almost comparable increase in the orienting potential associated with the macroscopic alignment. This is consistent with a freezing in of the smectic structure at this transition. The cage relaxation rate given by R ^{ c }, its ‘‘rotational diffusion coefficient,’’ is of order of 10^{7} s^{−1} in the I and N phases. It drops somewhat in the S _{ A } phase, but there is a greater than order of magnitude drop in R ^{ c } for the S _{ B } and C phases to about 10^{5} s^{−1}. This drop is also consistent with the freezing in of the smectic structure. The rotational diffusion tensor of the CSL probe is significantly larger than R ^{ c } which is consistent with the basic physical premise of the SRLS model. In particular, R _{⊥} ^{ o } and R _{∥} ^{ o } are of order 10^{8} s^{−1} and 10^{9} s^{−1} respectively.

Studies of spin relaxation and molecular dynamics in liquid crystals by two‐dimensional Fourier transform electron spin resonance. II. Perdeuterated‐tempone in butoxy benzylidene octylaniline and dynamic cage effects
View Description Hide DescriptionTwo‐dimensional Fourier transform (2D‐FT)‐electron spin resonance (ESR) studies on the small globular spin probe perdeuterated tempone (PDT) in the liquid crystalsolvent 4O,8 (butoxy benzylidene octylaniline) are reported. These experiments, over the temperature range of 95 °C to 24 °C, cover the isotropic (I), nematic (N), smecticA (S _{ A }), smecticB (S _{ B }), and crystal (C) phases. The 2D‐ELDOR (two‐dimensional electron–electron double resonance) spectra confirm the anomalously rapid reorientation of PDT, especially in the lower temperature phases. The model of a slowly relaxing local structure (SRLS) leads to generally very good non‐linear least squares (NLLS) global fits to the sets of 2D‐ELDOR spectra obtained at each temperature. These fits are significantly better than those achieved by the standard model of Brownian reorientation in a macroscopic orienting potential. The SRLS model is able to account for anomalies first observed in an earlier 2D‐ELDOR study on PDT in a different liquid crystal in its smectic phases. Although it is instructional to extract the various spectral densities from the COSY (correlation spectroscopy) and 2D‐ELDOR spectra, the use of NLLS global fitting to a full set of 2D‐ELDOR spectra is shown to be more reliable and convenient for obtaining optimum model parameters, especially in view of possible (incipient) slow motional effects from the SRLS or dynamic cage. The cage potential is found to remain fairly constant at about k _{ BT } over the various phases (with only a small drop in the S _{ B } phase), but its asymmetry increases with decreasing temperature T. This value is significantly larger than the weak macroscopic orienting potential which increases from 0.1 to 0.3k _{ BT } with decreasing T. The cage relaxation rate, given by R ^{ c } is about 3×10^{7} s^{−1} in the I phase, but increases to about 10^{8} s^{−1} in the S _{ A }, S _{ B }, and C phases. The rotational diffusion tensor for PDT shows only a small T‐independent asymmetry, and its mean rotational diffusion coefficient is of order 10^{10} s^{−1}, with however, a small increase in the S _{ B } phase. These results are consistent with a model previously proposed for PDT in benzylidene liquid crystalsolvents, that as T is reduced the PDT molecules are partially expelled from the hard core (dipolar) region of the liquid crystalline molecules toward the more flexible aliphatic chain region as a result of increased core packing from smectic layer formation, and it thus experiences a more fluid (for a given temperature) local cage structure.

An analysis of rotational transition probabilities and cross sections using close coupling, hard shape, and classical trajectory methods
View Description Hide DescriptionClassical and quantum transition probabilities were calculated applying the two (2D) and three (3D) dimensional close‐coupled and classical trajectory methods. The collision energy was 0.01 eV and, in this case, there are 12 classical accessible states for Xe−CO_{2} collisions. It was shown that for Δj≥8 the 2D and 3D transition probabilities agree quantitatively. The state‐to‐state total cross sections were also compared, in the same level of dimension, and the results showed similar trends. In addition, the hard shape quantum cross sections were compared with the 2D results and showed that the classical turning point cannot be used to obtain the semi‐axes of the ellipse, namely A and B. Moreover, the intensity of the hard shape cross sections showed to be different either for the 2D or 3D close coupling calculations. The comparison between the transition probabilities, carried out by the 2D and 3D quantum methods, showed the nature of the 2D results with accurate agreement for large Δj transitions.

Photodissociation of methylazide: Observation of triplet methylnitrene radical
View Description Hide DescriptionAn investigation of the photodissociation of methylazide at wavelengths from 292 nm to 325 nm is presented. Emission spectra and lifetime analysis show the existence of the triplet CH_{3}N radical. A simple kinetic model is proposed to explain the observed fluorescence real time profile, which reflects the vibrational relaxation of hot radical at high pressures. The photodissociation dynamics of methylazide seems complex. The predominant channel is likely to produce CH_{2}NH via concerted 1,2‐hydrogen shift and N_{2}extrusion process. The triplet CH_{3}N comes from a minor spin‐forbidden channel which involves possibly strong interaction between the low‐lying excited singlet and triplet states of methylazide.

Laser excited fluorescence study of reactions of excited Ca and Sr with water and alcohols: Product selectivity and energy disposal
View Description Hide DescriptionReactions of the metastable ^{3} P ^{0} _{ J } states of Ca and Sr in atomic beams with H_{2}O, D_{2}O, and CH_{3}OH yielding ground electronic state products have been observed by laser excited fluorescence of MOH, MOD, and MOCH_{3}. The water reactions favor metal hydroxide products while methanol reactions favor methoxides. For SrOH product, spectral simulation of the B̃ ^{2}Σ^{+}–X̃ ^{2}Σ^{+} transition based on coupled harmonic‐oscillator Franck–Condon factors was used to determine crude vibrational energy distributions in the bending and metal‐stretching modes, and simulation of a higher resolution scan of excitation of the ground vibrational level gave some information about the rotational energy distribution in that level. While excitation of metal stretching and rotation were considerable and not too far from the predictions of a prior model, bending was significantly colder. Limited spectroscopic constants and severe spectral congestion have precluded other successful simulations.

Microscopic description of nonadiabatic, nonequilibrium, and equilibrium solvations for solvated cluster reactions: (H_{2}O)_{ n }Cl^{−}+CH_{3}Cl→ClCH_{3}+Cl^{−}(H_{2}O)_{ n }
View Description Hide DescriptionA microscopic theory was presented for each of the nonadiabatic‐ and equilibrium‐solvation regimes in microsolvated cluster reactions to examine nonequilibrium‐solvation effects, and applied to the S _{ N }2 reactions: (H_{2}O)_{ n }Cl^{−}+CH_{3}Cl→ClCH_{3}+Cl^{−}(H_{2}O)_{ n } for n=0–4. To have pictures for nonadiabatic and equilibrium solvations, the potential‐energy surface of the reacting system on the transition‐state region was described with effective normal coordinates defined in each of these solvation limits. The solute dynamics in each of these solvation limits was considered to be determined by the effective frequencies characterizing the motions along the corresponding normal coordinates, and a rate‐constant expression was approximately derived. Ab initio molecular‐orbital calculations were carried out for the microsolvated S _{ N }2 reactions, and the ratio of nonadiabatic‐ to equilibrium‐solvation rate constants was evaluated. It was found that the ratio provides a better approximate value of a transmission coefficient that corresponds to the ratio of the nonequilibrium‐ to equilibrium‐solvation rate constants, for the larger values of number of microsolvated waters. It was supported that the nonadiabatic‐solvation picture appropriately characterizes the dynamics on the transition‐state region in such a reaction that the time scale of the reaction is very short compared to the motions of solvent reorganization. Furthermore, the finding that the transmission coefficients were quite small gave us a new understanding of the importance of the nonequilibrium‐solvation effect. In addition, the activation free energy for the microsolvated reaction in the case of n=4 was found unexpectedly to give most of the activation free energy for the corresponding solution reactions.

Vibrational predissociation of ArCl_{2}: Toward the determination of the potential energy surface of the B state
View Description Hide DescriptionAccurate quantum mechanical calculations are carried out to test the sensitivity of the spectroscopy and dynamics of the B state of ArCl_{2} to the steepness of the Morse term, α, of an atom–atom potential. It is discovered that the predissociationdynamics for this molecule are very complicated even in the Δv=−1 regime due to resonances in the continuum manifold of states. In both the Δv=−1 regime and the Δv=−2 regime the rate of vibrational predissociation and the product rotational distribution are extremely sensitive to the value chosen for α, but not in a regular way. For the Δv=−2 regime the variations can be attributed to spacings between resonances and the overlaps of the bright state wave functions with nearby dark states as expected from the intramolecular vibrational relaxation model. In the Δv=−1 regime, the variations are shown to originate from resonances in the v−1 continuum set of states. Although this makes it difficult to determine the value for α, a value of 1.8 Å^{−1} is probably close to the true value. The most useful new data to determine the potential would be measurements of the lifetimes for as many vibrational levels as possible and rotational distributions for excitation to low vibrational levels of the B state.

Short‐time photodissociation dynamics of A‐band and B‐band bromoiodomethane in solution: An examination of bond selective electronic excitation
View Description Hide DescriptionWe have obtained resonance Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A‐band and B‐band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time‐dependent wave packet calculations. Normal mode vibrational descriptions were used with the results of the calculations to find the short‐time photodissociation dynamics in terms of internal coordinates. The A‐band short‐time photodissociation dynamics indicate that the C–I bond becomes much longer, the C–Br bond becomes smaller, the I–C–Br angle becomes smaller, the H–C–Br angles become larger, the H–C–I angles become smaller, and the H–C–H angle becomes a bit smaller. The B‐band short‐time photodissociation dynamics indicate the C–Br bond becomes much longer, the C–I bond becomes slightly longer, the I–C–Br angle becomes smaller, the H–C–I angles become larger, the H–C–Br angles become smaller, and the H–C–H angle becomes slightly smaller. Both the A‐band and B‐band short‐time photodissociation dynamics appear to be most consistent with an impulsive ‘‘semi‐rigid’’ radical model qualitative description of the photodissociation with the CH_{2}Br radical changing to a more planar structure in the A‐band and the CH_{2}I radical changing to a more planar structure in the B band. We have carried out a Gaussian deconvolution of the A‐band and B‐band absorption spectra of bromoiodomethane, as well as iodomethane and bromomethane. The absorption spectra, resonance Raman intensities, and short‐time photodissociation dynamics suggest a moderate amount of coupling of the C–I and C–Br chromophores.

Preparation and probing of alignment in molecular ensembles by saturated coherent pulsed laser excitation
View Description Hide DescriptionAn analytical solution of the optical Bloch equations for a degenerate two‐level system is presented for coherent excitation with a monochromatic light pulse of rectangular time profile and linear polarization including off‐resonance interaction. The generalization to an arbitrary excitation and detection geometry is achieved by incorporation of the appropriate coordinate frame rotations. In this way the formalism can be applied to the determination of alignment parameters in molecular ensembles even under partially saturated conditions. In the limit of linear absorption the results reduce to those obtained by Greene and Zare [J. Chem. Phys. 78, 6741 (1983)]. For complete saturation on lines of a Q or R branch, polarized fluorescence detection is sensitive only to the quadrupole alignment moment. The formalism also allows investigation of the production of aligned ensembles via coherent optical pumping with a single pulse. Depending on the degree of saturation as well as the detuning from resonance, strong alignment can be created.