Single Rotational State Research Articles

Nuclear quadrupole quantum beat spectroscopy in the electronic ground state of a polyatomic molecule by an IR-UV double resonance method

The feasibility of determining nuclear quadrupole hyperfine splittings in the ground electronic state of a polyatomic molecule using a double resonance quantum beat method has been demonstrated. Pyrimidine molecules were coherently excited into single rotational states of the S 0 20 1 (13a 1) vibrational level. The resulting hyperfine level coherences were then probed, after a variable time delay, by excitation to the S 1 O 0 level with subsequent fluorescence detection. Analysis showed that the quadrupole structure in the 20 1 (13a 1) level is similar to that previously measured in the vibrational ground state. Applications of this method and possible experimental extensions are discussed.

State-to-state relaxation of highly vibrationally excited acetylene by argon

Vibrational overtone excitation followed by laser-induced fluorescence (LIF) interrogation determines state-to-state rotational and vibrational relaxation rates of highly energetic acetylene molecules by argon. We initially excite a single rotational state of the vibration 3ν3 (three quanta of C–H stretch, Evib=9640 cm−1) and detect the state (3ν3, Jf=20) in rotational relaxation measurements or the state (ν1+ν2+ν3+2ν4, l=0, Jf=4) in vibrational relaxation measurements. The data, which we acquire under single-collision conditions, show that argon rotationally relaxes acetylene almost half as efficiently as acetylene itself but is relatively inefficient in vibrational relaxation.

Rotational energy transfer in vibrationally excited acetylene X̃ 1Σg(ν2″=1,J″):ΔJ propensities

A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene–acetylene collisions at room temperature under single collision conditions. Initial rotational states (Ji=5,7,...,25) were prepared and final states (Jf=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong Q-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the à 1Au(ν3′ = 1)←X̃ 1Σg(ν2″ = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. In addition to this general behavior a significant ΔJ propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the ΔJ propensities, to the rate constant matrix is obtained, again with a single set of parameters.

Correlations between angular momentum orientation and exit velocity in gas–surface scattering: A probe of the dependence of collision dynamics on the position of impact

The scattering of rotationally cold N2 from Ag(111) results in angular momentum alignment and orientation of the scattered molecules; measurement of the angular momentum polarization as a function of exit angle, final J state, and exit translation energy provides direct information on the dynamics of the collisions. In this paper, the orientation of the angular momentum vector of the scattered N2 molecules, A{1}1−(J) has been measured for slow, medium, and fast groups of molecules in single rotational states at fixed exit angles. With normal incidence scattering (θi=0°) and off-normal detection, for a given final J state, the ‘‘slow’’ molecules have a higher probability of tumbling backwards (‘‘back spin’’) than the ‘‘fast’’ molecules. Conversely, for glancing incidence scattering (θi=30°) with quasi-specular detection, the opposite trend is observed: the slow molecules have a higher probability of tumbling forwards (‘‘top spin’’) than the fast molecules. These experiments were simulated and analyzed using molecular dynamics trajectory calculations. The calculations show that the amount of gas kinetic energy transferred to the surface is sensitive to the narrow dispersion of impact sites and molecular orientations that lead to scattering into a given final rotational state at a given exit angle. The calculations demonstrate that for both incident angles, collisions near the top of a surface atom lead to slower final velocities than collisions with the hollow sites in analogy with the simple case of two colliding spheres. Therefore, the experimentally observed dependence of the angular momentum orientation on the exit velocity results from the correlation between the initial molecular bond angle and the impact site for scattering into a given J state and at a fixed exit angle.

Photodissociation from an in-plane rotation in water as a direct probe of dynamics

Abstract The state-to-state photodissociation at 193 nm of the initially prepared in-plane rotational state, 3 03 , of the fundamental symmetric stretch of water, H 2 O, (1,0,0), is studied. Stimulated Raman excitation and coherent anti-Stokes Raman scattering prepare and detect respectively the single rotational state of H 2 O (1,0,0). Laser-induced fluorescence and Doppler polarization spectroscopy determine the rotational distribution and the vector correlations respectively of the OH photofragment resulting from the photodissociation of the specific rovibrational state. The distribution is structured and the Λ-doublet ratio in the two spin-orbit states shows preference of the A″ component. The correlations are close to the maximum attainable values expected for an idealized orientation in which the transition dipole moment of the parent is parallel to the fragment angular momentum and perpendicular to its velocity. This shows that experiments which prepare the parent molecule in a particular state before a second laser dissociates it provide a powerful means for molecular dynamics.

Direct measurement of thermal rate constants for state-to-state rotational energy transfer in collisions of CN(X 2Σ+, v=2, N) with He

Stimulated emission pumping state preparation and laser induced fluorescence state detection in the CN(B 2Σ+–X 2Σ+) violet system have been employed to study relaxation of single rotational states of CN(X 2Σ+, v=2) in collisions with He at 295 K. Approximately 2/3 of the value of the total removal rate constant for CN(X 2Σ+, v=2, N=2, 11, or 14) corresponds to changes in the rotational quantum number of ‖ΔN‖≤3 and a strong propensity is evident which favors even changes in N for ‖ΔN‖≤4. The measured rate constants are consistent with detailed balance, can be described with statistical power-gap and exponential energy-gap fitting functions and, where compared, are in accord with an IOS-based scaling function. No significant difference is found between a sum of state-to-state rate constants and separately measured total removal rate constants for N=2, 11, and 14. The total removal rate constants decrease monotonically between N=0 and N=41 to about 1/3 of their value at N=0.

Energy transfer in highly vibrationally excited acetylene: Relaxation for vibrational energies from 6500 to 13 000 cm−1

Vibrational overtone excitation of acetylene molecules to energies between 6500 and 13 000 cm−1 followed by interrogation of the excited states during collisional relaxation determines both the mechanism and rates of energy transfer. A pulsed visible or near-infrared laser excites a single rotational state of C2H2 in the region of the first (2νCH), second (3νCH), or third (4νCH) overtone of the C–H stretching vibration, and an ultraviolet laser probes the excited molecules by laser-induced fluorescence after a variable delay. The self-relaxation rate constant of about 9×10−10 cm3 molecules−1 s−1 is almost twice the Lennard-Jones collision rate constant and is nearly invariant with vibrational level. The energy-transfer rate constants from these population transfer measurements agree with those extracted from pressure-broadening data in both their size and insensitivity to vibrational state. Relaxation by the rare-gas atoms He, Ar, and Xe is nearly half as efficient as self-relaxation, suggesting that the internal structure of the collision partner is not particularly important in determining the relaxation rate. The invariance with vibrational level and the efficiency of rare-gas quenching indicate that rotational energy transfer is the most important relaxation pathway.

Product energy partitioning in the unimolecular decomposition of vibrationally and rotationally state-selected hydrogen peroxide

Infrared-optical double resonance prepares HOOH molecules in single rotational levels of the 6νOH, 5νOH+νOOH, 5νOH+νOO, and 4νOH+νOH′ vibrational states which range from 3 to 2287 cm−1 of excess energy above the unimolecular dissociation threshold. Laser-induced fluorescence probes the nascent OH rotational state distributions from the decomposition of rovibrationally selected reactants. The nascent rotational state distributions reveal that both OH spin–orbit states can be populated by the decomposition of a single molecule and hence that electronic angular momentum is not conserved throughout the dissociation process. The product state distributions from reactants excited to the 6νOH and 4νOH+νOH′ vibrational levels are generally in good agreement with the predictions of phase-space theory provided electronic angular momentum is treated statistically. Reactants decomposing from single rotational states in the 5νOH+νOOH combination level (and to a lesser extent the 5νOH+νOO level) show product state distributions which are systematically colder than phase-space theory predictions. This observation indicates that energy redistribution in vibrationally excited HOOH is not complete on the time scale of unimolecular decomposition.

Predissociation of the NH/ND(c 1Π,v′,J′) states

The predissociation of the electronically excited NH/ND(c 1Π) state was studied by accurate lifetime measurements. The pulsed generation of single rotational states of different vibrational levels was performed by dye laser excitation in the NH/ND c 1Π–a 1Δ band system. The lifetime of ND(c,v′=0,J′=1) is 500±10 ns. This value is close to the purely radiative lifetime of the rotationless state. The lifetime increases steadily up to J′=15. The lifetime of ND(c,v′=1) is 235 ns for very low J′ and decreases for higher J′. These shorter lifetimes are due to predissociation by the repulsive 1 5Σ− state. Similarly, the NH(c,v′=0) state is predissociated for all J′, having an average lifetime of 463 ns for J′=1–7. For J′>7, the lifetime decreases steadily. The lifetime of NH(c,v′=1) is dominated by predissociation, with values near 65 ns for J′=1–6. These experimental results are found to be in very good agreement with an ab initio electronic structure calculation of the radiative and predissociative decay rates.

Complete description of two-photon (1+1′) ionization of NO deduced from rotationally resolved photoelectron angular distributions

Time-of-flight photoelectron spectroscopy has been used to record energy-resolved photoelectron angular distributions (PADs) following (1+1′) resonance-enhanced multiphoton ionization (REMPI) of NO via the vi=1,Ni=22 rovibrational level of the A 2∑+ state. The PADs corresponding to single rotational states of the resulting molecular ion show a strong dependence on the change in ion core rotation ΔN(≡N+−Ni) and also on the angle between the linear polarization vectors of the two light beams. Broken reflection symmetry [I(θ,φ)≠I(−θ,φ)] is observed when the polarization vectors of the two light beams form an angle of 54.7°. A fit to the PADs provides a complete description of this molecular photoionization, namely, the magnitudes and phases of the radial dipole matrix elements that connect the intermediate state to the ‖lλ〉 photoelectron partial waves (Refs. 1 and 2). This information is then used to predict unobserved quantities, such as ion angular momentum alignment and the full three-dimensional form of the PADs.

Direct measurements of collision-induced state-to-state rotational energy transfer rates in C 2H 2 (ν″ 2 = 1)

We report the direct measurement of state-to-state rotational relaxation rates in the IR-inactive (ν″ 2 = 1) vibrational mode of acetylene at room temperature. Stimulated Raman pumping in the Q-branch of the (ν″ 2 = 1) ← (ν″ 2 = 0) transition is used to prepare a significant population in single rotational states of the upper vibrational level. The population in this level is probed by laser-induced fluorescence via the à 1A u(ν′ 3 = 1) ← X̃ 1∑ + g(ν″ 2 = 1) electronic transition. Rotational relaxation rates are derived from the collisional redistribution of the population in the range between 0.02 and 0.4 hard-sphere collisions. This low collision number enables us directly to determine the collision matrix for single and multiple quanta transitions, which account for approximately 40–50% of the population loss in the initially prepared state. For J″ = 9, 11, and 13, total depopulation rate constants of about (8.8±1.0) × 10 −10 cm 3 s −1 ( J″ = 11, 13) are observed.

State resolved photodissociation of vibrationally excited water: Rotations, stretching vibrations, and relative cross sections

The state resolved photodissociation of highly vibrationally excited water molecules using laser induced fluorescence detection of the OH product demonstrates the control that the initially selected state exerts over product state populations. These vibrationally mediated photodissociation experiments, in which one photon prepares a highly vibrationally excited molecule and a second photon dissociates it, determine the role of overall rotations and of O–H stretching vibrations as well as measure the relative cross section for the photodissociation of water. The rotational state of the vibrationally excited water molecule governs the rotational state of the OH product of the dissociation, in agreement with ab initio calculations and previous measurements on single rotational states excited in the fundamental asymmetric stretching vibration band. The initially selected vibrational state of the water molecule determines the vibrational energy disposal in the products, which agrees with a simple qualitative model based on the pattern of the initially selected vibrational wave function. Dissociating vibrational states with similar energies but very different nuclear motions produces dramatically different product vibrational state populations. The vibrational energy initially present in the surviving bond primarily appears as vibrational excitation of the product. Dissociation of the ‖04〉− state produces no vibrationally excited OH, but dissociation of the ‖13〉− state produces mostly vibrationally excited products. These qualitative notions agree well with recently detailed ab initio calculations. The relative photodissociatiuib cross section of the highly vibrationally excited molecule shows structure over the wavelength range of 218.5 to 266 nm that reflects the nodal pattern of the intermediate vibrational state in the dissociation and confirms the predictions of theoretical calculations.

Unimolecular dissociation of hydrogen peroxide from single rovibrational states near threshold

Infrared-optical double resonance excitation of hydrogen peroxide using the ν3+ν5 and ν2 + ν5 combination bands as intermediate levels prepares reactant molecules in single rotational states near the OO dissociation threshold. The band origins of the ΔvOH = 4 vibrational overtone transitions originating from these combination bands provide information on the anharmonicities between the OH stretch and the OO stretch and OOH bend, respectively. At low resolution the vibrational overtone transitions are clearly parallel bands of a near prolate symmetric top and can be assigned to zeroth-order J and K quantum numbers. At 0.1 cm−1 resolution the individual features in the vibrational overtone spectra appear as clumps of sharp lines centered at the frequencies of the zeroth-order symmetric top transitions. The number of components within a clump appears to be less than the total number of available vibrational states. The narrowest feature observed has a linewidth of 0.12 cm−1 and sets a lower bound of ∼35 ps on the lifetime of the dissociating molecule. As J increases from 1 to 21, the clumps of lines coalesce into a smooth Lorentzian envelope. The overall clump width decreases with J, reaching an asymptotic value of 0.67 cm−1 at J≂17. A model incorporating a J dependent density of bath levels that couple to the zeroth-order bright state predicts the observed changes in the vibrational overtone transitions with J. The rotationally resolved vibrational overtone spectra provide information on the time scale for the conservation of K of the dissociating molecule.

Rotationally resolved vibrational overtone spectroscopy of hydrogen peroxide at chemically significant energies

An infrared–optical double resonance scheme simplifies the room temperature 6νOH vibrational overtone spectrum of hydrogen peroxide and prepares highly excited reactant molecules in single rotational states for unimolecular reaction studies. First, an optical parametric oscillator excites the OH asymmetric stretch (ν5) and selects a single or small subset of rotational states. A visible dye laser pulse then promotes molecules from vOH=1 to vOH=6 where they subsequently dissociate to produce two OH fragments. A third laser detects the dissociation products via laser induced fluorescence. The rotationally resolved vibrational overtone spectra of hydrogen peroxide generated by scanning the visible dye laser frequency are assignable to a parallel band of a near prolate symmetric top. Linewidths of the individual rovibrational features range from 1–3 cm−1 but show no systematic dependence upon the rotational quantum numbers and are attributed predominantly to anharmonic coupling of the zeroth-order bright state to dark background states. The assignability of the double-resonance vibrational overtone spectra to J and K quantum numbers implies that K is conserved for at least a time determined by the linewidth of a single zeroth-order rovibrational feature.

Glass transition in polymers: freezing of rotational oscillations

A nonlattice model based on space-filling criteria is adopted to investigate the glass transition in polymers. In the glass phase the chains are considered to assume a single rotational isomeric state. The temperature T 2 at which the entropies of the amorphous and of the glass phase are equal is taken as the lower limit of the glass transition temperature for slow experiments. Calculations performed on polyethylene, iso- and syndiotactic polypropylene, atactic polystyrene, cis-1,4-polybutadiene, and cis-1,4-polyisoprene indicate that a substantial reduction of the average rotational oscillation around single bonds takes place upon lowering the temperature at T 2

Theoretical investigation of collision induced rotational alignment in N+2–He

Collision induced rotational alignment of N+2 ions drifting in a helium buffer gas is studied by quantum closed coupled calculations using an ab initio interaction potential obtained from multireference configuration interaction wave functions. New formulas are derived for the tensor cross sections. For a given velocity distribution of the collisional partners a set of kinetic equations is solved under steady-state conditions. The resulting alignment parameters are found to be smaller than the experimental values for the velocity distribution assumed so far in drift tube experiments. However, by modification of the anisotropy of this distribution, good agreement between the theoretical quadrupole moments of the rotational angular momentum distributions and the corresponding experimental data can be obtained. It is shown that the attractive part of the potential has a significant influence on the collision dynamics of the N+2–He system. The closed coupled m-resolved cross sections indicate that collision induced transitions between magnetic sublevels of a single rotational state contribute more to the alignment effect than transitions between different rotational states.

Single rotational state preparation and relaxation in CN(X 2Σ +, ν=2). Stimulated emission pumping and laser-induced fluorescence detection

Stimulated emission pumping (SEP) in the CN(B 2Σ +−X 2Σ +) violet system has been employed to prepare substantial populations of CN(X 2Σ +, ν=2) molecules in single rotational states. These populations are detected by laser-induced fluorescence (LIF) and the combined SEP-LIF procedure is used to observe the population decay of the prepared states in collisions with helium atoms.

Dynamic Behavior of Individual Rovibronic States in S1 Benzene

AbstractPulsed Doppler‐free two‐photon excitation of single rotational states of the 141 vibronic state of benzene with a resolution of 70 MHz is employed to investigate the influence of molecular rotation on the decay behavior of these states. The analysis of the completely resolved (Δv = 15 MHz) spectrum of the 1410 band, measured with a CW laser, makes it possible to specify whether individual states are perturbed by other (“dark”) S1 states and allows the precise determination of the degree of admixture of these background states. The vibrational identity of the background states is known from emission spectra of single rovibronic states. Unperturbed rovibronic states were investigated up to J'K' = 6761, and a pure exponential decay was found which does not depend on molecular rotation in accord with a nonradiative decay of S1 benzene in the statistical limit. The decay of perturbed states, which was also found to be purely exponential, depends strongly on the degree of admixture of the background state and on its vibrational identity. Values of the decay time of two identified “dark” zero‐order S1 vibrational states, which cannot be directly excited by optical transitions, are determined by evaluation of the decay behavior of the perturbed states.

Rotationally resolved spectra of the 61 and 6111 band of benzene in a moderately cold molecular beam: Spectral and dynamical analysis

Rotationally resolved spectra of the 610 and 610110 band of benzene, C6H6, in a supersonic molecular beam at rotational temperatures between 8 and 50 K are reported. The spectra consist of lines of 85 MHz linewidth. An unexpectedly low saturation intensity of ≊104 W/cm2 is found for the observed one-photon transitions, and it is shown that the saturation intensity differs for different rotational lines within one vibronic band. The rovibronic line spectra are analyzed within the framework of a rigid symmetric top model and highly precise values of the rotational constants are determined. In addition, the rotationless transition frequencies ν00 are obtained with high precision. The spectrum of the 610 band shows no signs of rotational perturbations, while the 610110 band at higher vibrational excess energy shows indications of perturbations for lines with K′ above 10. The decay times of single rotational states within the 6111 vibronic state are reported and no rotational dependence of the decay time is found in agreement with the statistical limit character of the interstate nonradiative process.

Vibrationally mediated photodissociation of t-butyl hydroperoxide: Vibrational overtone spectroscopy and photodissociation dynamics

Vibrationally mediated photodissociation is a two-photon technique for studying the spectroscopy and photodissociation dynamics of highly vibrationally excited molecules. In these experiments, a highly vibrationally excited t-butyl hydroperoxide (t-BuOOH) molecule, prepared by excitation in the region of the third overtone of the O–H stretching vibration (4νOH), absorbs a second photon to dissociate to OH and t-butoxy fragments, and laser induced fluorescence determines the quantum state populations of the OH fragment. Vibrational overtone excitation spectra, obtained by varying the vibrational overtone excitation wavelength while monitoring a single OH rotational state, are nearly identical to photoacoustic spectra. We fit the coarse structure in the vibrational overtone excitation spectrum in the region of the 4νOH transition and the photoacoustic spectra in the regions of the 5νOH and 6νOH transitions using a spectroscopic model of the interaction of the O–H bond stretching vibration with the torsional vibration about the O–O bond. This analysis determines the barrier to internal rotation of the O–H and t-butoxy groups through the trans configuration and its variation with vibrational excitation. The trans barrier in the ground vibrational state is 275 cm−1 and increases with vibrational excitation to 425, 575, and 680 cm−1 for t-BuOOH molecules with four, five, and six quanta of O–H stretching excitation, respectively. Comparison of the energy disposal in the vibrationally mediated photodissociation with that for direct photolysis at 376 nm, which adds the same amount of energy to the molecule, illustrates the unique dynamics that can occur when vibrational excitation precedes photodissociation. Single-photon photolysis produces fragments with large recoil velocities, while vibrationally mediated photodissociation produces slowly recoiling fragments having substantially more energy in internal excitation.