Zeeman Quantum Beat Spectroscopy Research Articles

Experimental and theoretical studies of the Xe-OH(A/X) quenching system.

New multi-reference, global ab initio potential energy surfaces (PESs) are reported for the interaction of Xe atoms with OH radicals in their ground X2Π and excited A2Σ+ states, together with the non-adiabatic couplings between them. The 2A' excited potential features a very deep well at the collinear Xe-OH configuration whose minimum corresponds to the avoided crossing with the 1A' PES. It is therefore expected that, as with collisions of Kr + OH(A), electronic quenching will play a major role in the dynamics, competing favorably with rotational energy transfer within the 2A' state. The surfaces and couplings are used in full three-state surface-hopping trajectory calculations, including roto-electronic couplings, to calculate integral cross sections for electronic quenching and collisional removal. Experimental cross sections, measured using Zeeman quantum beat spectroscopy, are also presented here for comparison with these calculations. Unlike similar previous work on the collisions of OH(A) with Kr, the surface-hopping calculations are only able to account qualitatively for the experimentally observed electronic quenching cross sections, with those calculated being around a factor of two smaller than the experimental ones. However, the predicted total depopulation of the initial rovibrational state of OH(A) (quenching plus rotational energy transfer) agrees well with the experimental results. Possible reasons for the discrepancies are discussed in detail.

An experimental study of OH(A2Σ+) + H2: Electronic quenching, rotational energy transfer, and collisional depolarization.

Zeeman quantum beat spectroscopy has been used to determine the thermal (300 K) rate constants for electronic quenching, rotational energy transfer, and collisional depolarization of OH(A2Σ+) by H2. Cross sections for both the collisional disorientation and collisional disalignment of the angular momentum in the OH(A2Σ+) radical are reported. The experimental results for OH(A2Σ+) + H2 are compared to previous work on the OH(A2Σ+) + He and Ar systems. Further comparisons are also made to the OH(A2Σ+) + Kr system, which has been shown to display significant non-adiabatic dynamics. The OH(A2Σ+) + H2 experimental data reveal that collisions that survive the electronic quenching process are highly depolarizing, reflecting the deep potential energy wells that exist on the excited electronic state surface.

A new potential energy surface for OH(A 2Σ+)–Kr: The van der Waals complex and inelastic scattering

New ab initio studies of the OH(A(2)Σ(+))-Kr system reveal significantly deeper potential energy wells than previously believed, particularly for the linear configuration in which Kr is bound to the oxygen atom side of OH(A(2)Σ(+)). In spite of this difference with previous work, bound state calculations based on a new RCCSD(T) potential energy surface yield an energy level structure in reasonable accord with previous studies. However, the new calculations suggest the need for a reassignment of the vibrational levels of the electronically excited complex. Quantum mechanical and quasi-classical trajectory scattering calculations are also performed on the new potential energy surface. New experimental measurements of rotational inelastic scattering cross sections are reported, obtained using Zeeman quantum beat spectroscopy. The values of the rotational energy transfer cross sections measured experimentally are in good agreement with those derived from the dynamical calculations on the new adiabatic potential energy surface.

Molecular photofragment orientation in the photodissociation of H2O2 at 193 nm and 248 nm

Angular momentum orientation has been observed in the OH(X(2)Π, v = 0) fragments generated by circularly polarized photodissociation of H(2)O(2) at 193 nm and 248 nm. The magnitude and sign of the orientation are strongly dependent on the OH(X) photofragment rotational state. In addition to conventional laser induced fluorescence methods, Zeeman quantum beat spectroscopy has also been used as a complementary tool to probe the angular momentum orientation parameters. The measured orientation at 193 nm is attributed solely to photodissociation via the Ã(1)A state, even though at this wavelength H(2)O(2) is excited near equally to both the Ã(1)A and B(1)B electronic states. This observation is confirmed by measurements of the photofragment orientation at 248 nm, where access to the Ã(1)A state dominates. Several possible mechanisms are discussed to explain the observed photofragment orientation, and a simple physical model is developed, which includes the effects of the polarization of the parent molecular rotation upon absorption of circularly polarized light. Good agreement between the experimental and simulation results is obtained, lending support to the validity of the model. It is proposed that photofragment orientation arises mainly from the coupling of the parent rotational angular momentum with that induced during photofragmentation.

Applications of Zeeman quantum beat spectroscopy to angular momentum polarization studies

New applications of Zeeman quantum beat spectroscopy (ZQBS) to the study of angular momentum polarization effects produced by photodissociation or molecular collisions are introduced. A brief account of recent work on measuring the collisional depolarization of OH(A2Σ+) by Ar demonstrates the usefulness of ZQBS as a tool for studying angular momentum polarization effects. Results are presented for the polarization of the NO(X) photofragments generated by the photolysis of NO2 via its first absorption band at wavelengths around 300 and 350 nm. These experiments illustrate the feasibility of measuring a wide range of vector correlations using ZQBS, and point the way to a number of other potential applications of the technique.

Elastic Depolarization of OH(A) by He and Ar: A Comparative Study

Two color polarization spectroscopy has been employed to measure the collisional depolarization of OH(A(2)Sigma(+), v = 1) by He and Ar. Complementary experiments using Zeeman quantum beat spectroscopy have also been performed to determine separately the cross sections for rotational energy transfer (RET) out of selected rotational levels of OH(A, v = 0) + Ar, as well as those for elastic depolarization. This has been achieved by dispersing the emission, so as to observe a single fluorescence transition. Elastic depolarization of OH(A) by Ar is found to be significant with that for loss of rotational alignment exceeding that for loss of orientation. In the case of OH(A) + He, the polarization spectroscopy measurements suggest that elastic depolarization plays a relatively minor role in the loss of the polarization signal compared with RET. The experimental data for OH(A) + Ar are compared in detail with the results of quasi-classical trajectory calculations that accommodate the effects of electron spin. These classical calculations are assessed against the results obtained using full close-coupled open shell quantum mechanical scattering methods. Overall the level of agreement between the two experiments, and between experiment and theory, is very reasonable. Surprisingly, at low N the elastic depolarization cross sections for OH(A) + Ar are found to be quite similar in magnitude to those observed for OH(X) + Ar despite the fact that the well depth in the latter system is considerably smaller than that for OH(A)-Ar. However, for OH(A) + Ar the depolarization cross sections are insensitive to N in the range 1-14. It is proposed that this behavior partly reflects the relatively anisotropic nature of the potential energy surface, which exhibits deep wells of different depths at the two linear configurations OH(A)-Ar and Ar-OH(A), and partly the nature of elastic depolarizing collisions, which must occur with a velocity component perpendicular to the plane of rotation of the diatomic molecule.

Collisional depolarization of OH(A) with Ar: Experiment and theory

Zeeman quantum beat spectroscopy has been used to measure the 300 K rate constants for the angular momentum depolarization of OH(A (2)Sigma(+)) in the presence of Ar. We show that the beat amplitude at short times, in the absence of collisions, is well described by previously developed line strength theory for (1+1) laser induced fluorescence. The subsequent pressure dependent decay of the beat amplitude is used to extract depolarization rate constants and estimates of collisional depolarization cross sections. Depolarization accompanies both inelastic collisions, giving rise to rotational energy transfer, and elastic collisions, which change m(j) but conserve j. Previous experimental studies, as well as classical theory, suggest that elastic scattering contributes around 20% to the observed total depolarization rate at low j. Simulation of the experimental beat amplitudes, using theoretical calculations presented in the preceding paper, reveals that depolarization of OH(A) by Ar has a rate constant comparable to, if not larger than, that for energy transfer. This is consistent with a significant tilting or realignment of j(') away from j on collision. The experimental data are used to provide a detailed test of quantum mechanical and quasiclassical trajectory scattering calculations performed on a recently developed ab initio potential energy surface of Kłos et al. [J. Chem. Phys. 129, 054301 (2008)]. The calculations and simulations account well for the observed cross sections at high N, but underestimate the experimental results by between 10% and 20% at low N, possibly due to remaining inaccuracies in the potential energy surface or perhaps to limitations in the dynamical approximations made, particularly the freezing of the OH(A) bond.

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

The depolarization of the rotational angular momentum of electronically excited OH(2Σ) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s-1. The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach ∼100 Å2, only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by significant depolarization. The cross-section for the translational moderation of superthermal OH(X) by water is also determined in the present study.

Zeeman quantum-beat spectroscopy of NO2: Eigenstate-resolved Landé gF factors near dissociation threshold

The sign and magnitude of Landé gF factors for single NO2 rovibronic (J=3/2) eigenstates in the 15 cm−1 region below dissociation threshold (D0=25 128.57 cm−1) were investigated using Zeeman quantum-beat spectroscopy. The derived Landé gF factors exhibit pronounced fluctuations about an average much smaller than expected in the absence of rovibronic perturbations, which destroy the goodness of the N and K quantum numbers and the J=N+S coupling scheme. The F=J+I coupling scheme was found to be valid near D0 to within the uncertainty of our measurements, and the average Landé gF factors near dissociation threshold are in good agreement with those calculated under the assumption of complete rovibronic mixing. Our findings do not provide evidence for the participation of repulsive quartet states near dissociation threshold.

Zeeman quantum beat spectroscopy of higher rovibrational levels of the V 1B 2 state of carbon disulfide

Lifetimes and g values are measured for low J rotational levels of eight of the higher vibrational states of V 1B 2 CS 2. The measurements are conducted in a supersonic jet using the Zeeman quantum beat technique. All fluorescence decays are biexponential with a shorter-lived component (≈ 2 μs) corresponding to discrete emission and a longer-lived component (≈ 10 μs) corresponding to continuum-like emission. Only the discrete component shows quantum beats in the presence of a magnetic field. The g values range from 0.005 to 0.040 with no apparent dependence on rotational or vibrational quantum numbers. The large size of these g values can be accounted for by a model in which the V state levels acquire triplet character through spin-orbit interaction with nearby levels possibly belonging to the R 3A 2 state.

Rovibronic level structure of electronically excited acetylene ( [formula omitted]1A u) in a supersonic jet as studied by laser-induced fluorescence and Zeeman quantum beat spectroscopy

Laser-induced fluorescence (LIF) spectra of the A 1A u- X 1Σ g + transition of jet-cooled acetylene were measured resolution to resolve the respective rovibronic levels in the A nv′ 3 vibrational states. Most of rotational levels of the nv′ and 4) state split into several levels, while level splitting occurs scarcely for those in the 2 v′ 3 state. The observed radiative lifetimes of the respective levels are different from each other. Especially, the average lifetime of levels in the 3 v′ 3 state is relatively long compared with those in the other states. The fluorescence from a number of levels decays in an oscillatory manner under a weak magnetic field. This phenomenon is attributed to a coherent interaction between the Zeeman sublevels, from which a magnetic moment, i.e., a g-factor, can be determined characteristic to each level. A large g-factor assigned to a level implies that the level couples strongly with a triplet state. Long lifetimes and large g-factors found for levels in the 3 v′ 3 state imply that these levels couple with triplet states more efficiently than other levels in the 4 v′ 3 or 2 v′ 3 state. Besides the Zeeman quantum beat, a quantum beat ascribed to level anticrossing between a level in the A nv′ 3 state and a nonfluorescent level is found for a number of excited levels. These anticrossings are due to interaction with its strength of the order of MHz, and ascribed to couplings with vibrationally highly excited levels in the X state.

Rotationally resolved laser-induced fluorescence and zeeman quantum beat spectroscopy of the V 1B 2 state of jet-cooled CS 2

The rotationally resolved laser-induced fluorescence (LIF) excitation spectrum of V system bands (V 1B 2≈X 1Σ 1 g transition) of CS 2 cooled in a supersonic jet has been observed. In a supersonic jet of CS 2/Ar or He mixture, the rotational temperature of CS 2 is reduced to less than 10 K, and thus the LIF excitation spectrum is simplified significantly. Two types of rotational structure are found; one is composed of P and R branch transitions from even J″ levels and the other is of P, Q, and R branch transitions from even as well as J″. The bands with the former rotational structure are assigned to transitions to K′ = O levels of 1B 2 state, the bands with the latter structure to transitions to K′ = 1 levels from the (O, 1 1, O) level of the electronic ground state, i.e. vibrationally hot bands. This assignment is supported by the further evidence that these hot bands disappear when the supersonic jet includes a third-body gas such as NH 3 which enhances the vibrational relaxation of CS 2. Calculation of transition moments for respective leads to the conclusion that the upper levels of the V system bands are located in the region close to or higher than the potential barrier of the bending vibration of excited CS 2. The radiative lifetime of CS 2 in single rovibronic levels of the 1B 2 state is in the range of 2–8 μs which is of the same order of magnitude as that calculated from the absorption coefficient. It tends to be longer for higher J levels or for higher vibronic levels. Zeeman quantum beating is observed in the fluorescence decay of excited CS 2 for a number of rovibronic levels under a weak magnetic field, and thus a magnetic moment associated with each rovibronic level can be determined. The g values are around 0.02 and tend to be smaller in higher J levels for some vibronic states. Based on the the observed radiative lifetime and the g value, it is suggested that the 1B 2 state is perturbed by a spin-rotation interaction with two spin components, A 1 and B 1 of the 3A 2 orbital state besides a strong spin-orbit coupling with the R 3B 2 state.

Collisional quenching and depolarization of NO2 Ã 2B2 state fluorescence as studied by Zeeman quantum beat spectroscopy

We have studied the effect of collisions on the time-resolved fluorescence arising from a single optically selected fine structure level (F1303) of the à 2B2 state of NO2 prepared via absorption near 5933 Å in the presence of a weak magnetic field. The fluorescence modulation induced by the magnetic field (Zeeman quantum beats) is used to monitor the time dependence of the alignment. The alignment decays exponentially and follows simple Stern–Volmer kinetics. The incoherent fluorescence intensity, used to determine population information, fits well a bi-exponential form at nonzero pressure. This is interpreted by a simple kinetic scheme involving cascade to additional fluorescing states. We find that only about 15% of the population removed from the initial level by collision produces subsequent emission that is detected (630–830 nm). We are unable to identify the exact nature of these emitting states. However, it is clear that pure rotationally inelastic collisions do not dominate the energy transfer process. Collisional population and alignment relaxation rate constants for the optically prepared level have been measured for excited state collisions with He, Ne, Ar, Kr, Xe, N2, CO, and NO2 at 295 K and low density (<1×1015 particle/cm3). In particular, we find that the rate for population removal of the initially excited level by collision is faster than the rate for elastic disalignment (depolarization within the same rotational level).