Predissociation Rates Research Articles

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Photofragment-excitation spectrum of NO 2 observed through O( 3P J) detection

Photodissociation dynamics of NO 2 was investigated by observation of the photofragment-excitation (PHOFEX) spectrum of NO 2 obtained by detecting O ( 3P j , J = 2, 1, and 0). Near the photodissociation threshold, the average branching ratios of three spin—orbit sublevels were consistent with those predicted by a restricted statistical distribution model, in which the predissociation rate to produce O atoms in each spin—orbit state is proportional to the limited number of energetically allowed states of the NO fragment.

Some Comments on the Temperature Measurement in a CO2 Laser Beam by Laser-Induced Fluorescence of O2: Improvement of the Method

In a recent paper, we used induced fluorescence of O2 excited by a broad-band ArF laser to measure the temperature in a CO2 laser beam. In this preliminary work, only the peak around 235 nm of the hot O2 laser-induced fluorescence spectrum was analyzed; it was calibrated by comparison with the same feature observed in the absence of the heating laser beam. The procedure, based on a comparison of experimental and simulated spectra, was very simple and efficient, but it suffered from several limitations. On one hand, it did not take advantage of the full experimental data set, the observed spectrum consisting of 5 peaks extending from 220 to 260 nm. More questionable was the calibration, since the two compared spectra recorded in the presence and in the absence of the CO2 laser beam were observed independently; therefore, the experimental conditions—in particular, the geometry of the absorbing zone—were not identical. On the other hand, the rotational variation in the predissociation rate in the B state of O2, which is, indeed, fairly large (in particular, for uB = 6, 7, 9), was neglected. In the present paper, we describe a more refined spectral analysis of the 230–260 nm region of the LIF spectrum. A comparison with the 300 K LIF spectrum is unnecessary when the spectral simulation analysis is extended to several peaks; the shape of the different features and the relative intensity provide internal consistency. The earlier neglect of the rotational variation of the B quantum yield resulted in an underestimation of the population of the higher vibrational levels (ux = 2, 3) and, therefore, of the temperature. Eventually, the actual temperature proved to be notably higher than our previous estimation (700 K).

Comment on “Predissociation of ammonia à state studied by microwave—optical double resonance spectroscopy”

The rotational dependence of the predissociation rate for NH 3 (Ã) v = 0 observed by Endo, Iida and Ohshima is shown to be fully compatible with earlier calculations based on the centrifugal modification of the effective barrier to dissociation.

Electronic structure and lifetime of an excimer state of HeF

Theoretical calculations have been carried out on the few lowest electronic states of HeF in an effort to identify a possible excimer state. The results show that the 2 2Σ+ state is bound with a minimum at 2.0 bohr. The calculated predissociation rates of the υ=0, N=1 level of the 2 2Σ+ state are lower by two orders of magnitude than the rate of the radiative transition 2 2Σ+, which has a vertical transition energy of 9.4 eV.

A search for the low-lying triplet state in ammonia by photoionization of a neutralized ion beam

A neutralized ion beam photoionization technique has been used in an attempt to detect the lowest triplet state in ammonia. Although triplet ammonia molecules are a likely product of the neutralization reaction, only highly excited Rydberg states were observed by photoionization. The absence of the triplet state is most likely due to its rapid predissociation rate.

High Resolution Laser Spectroscopy of Small Molecules and Clusters

The combination of high resolution laser spectroscopy in collimated cold molecular beams with mass selective detection allows investigations of rotationally resolved molecular spectra of selected molecular species in the presence of molecules with overlapping absorption spectra. This is illustrated by some examples, such as the isotope selective spectroscopy of Ag2–dimers or the sub-Doppler spectroscopy of Na3 in supersonic beam with a broad mass distribution of different clusters. Time resolved laser spectroscopy of excited states of alkali molecules, perturbed by bound dark states or by repulsive states, yields information about singlet–triplet mixing and predissociation rates.

Subpicosecond predissociation dynamics of the methyl radical Rydberg 3 s state

The subpicosecond dissociation dynamics of the methyl radical 3 s Rydberg state have been examined using rotational resonance Raman spectroscopy. The rovibronic dependence of the excited state predissociation rates and lifetimes are obtained for the B̃ state origin vibrational level of CH3 and CD3. Analysis of the tunneling rates using a cubic potential barrier yields estimates for the height (2200 cm−1) and position of the barrier along the dissociation coordinate (1.38 Å). A comparison of the potential energy surface parameters for the Rydberg 3 s states of methyl radical and ammonia is presented.

Predissociation of ammonia à state studied by microwave—optical double-resonance spectroscopy

Homogeneous linewidths of the individual rotational transitions in the ÃX̃ system of ammonia were determined precisely by a microwave—optical double-resonance technique, where a single inversion transition in the ground state was selected by microwave radiation, and subsequent electronic transitions were observed by changes in the microwave absorption. The linewidths, which directly correspond to the à state predissociation rates, increase with J but are nearly independent of K. Small but distinct asymmetry in the line shape was observed for the first time.

Predissociation of the Cs2 C 1Πu state

By scanning the frequency of a single-mode dye laser crossed to a molecular beam of Cs2, we measured the excitation spectra of the C 1∏u–X 1∑+g transition by detecting selectively the molecular fluorescence and the emission from the dissociated Cs(62P3/2) atoms. Dependence of the predissociation on each vibrational and rotational level C 1∏u (v,J) is studied by comparing the intensities of molecular fluorescence and atomic emission. This predissociation is found to depend strongly on the vibrational quantum number v but weakly on the rotational quantum number J, and to occur most strongly around the v=3 level. The Franck–Condon factors between the RKR potential curve of the C 1∏u state and several repulsive potential curves are calculated, and are compared with the observed predissociation rates. The potential curve of the (2)3∑+u state, which is expected to be repulsive and to cause the predissociation through the spin–orbit interaction, is estimated to cross the potential curve of the C 1∏u state between the left turning points of v=1 and v=0 levels.

Weak predissociation of the EF, GK, and H 1Σ+g states of the H2 molecule by nonadiabatic coupling with the electronic ground state

The energy shifts and widths generated in the vibrational levels of the EF, GK, and H 1Σ+g states of H2 by their nonadiabatic interactions with the electronic ground state have been calculated using ab initio wave functions. The shifts of the 44 quasibound vibronic levels of these excited states range from +0.02 to +1.00 cm−1. Along the EF progression the shifts are found to depend on vibrational kinetic energy in surprisingly good accord with the Van Vleck approximation. The calculated nonradiative decay probabilities range up to 3.3×106 s−1, and in one case the predissociation rate is as large as 39% of the radiative one. Experimental tests of these predictions are not yet available.

Ab initio study of the HeAr + ion: potential energy curves for the lowest states, spin-orbit coupling and predissociation rates

Multi-reference configuration interaction calculations are carried out for the three lowest states, X 2Σ +, A 2Π and B 2Σ +, of the HeAr + ion. The spin-orbit interaction between the Born-Oppenheimer states X 2Σ + and A 2Π is calculated up to second order by employing the Breit-Pauli Hamiltonian. The results are employed to construct the potentials for 2Π 1 2 and 2Π 1 2 states. Predissociation rates of the rotational levels in the 2Π 1 2 ( v=0) state are also calculated on the basis of the computed ab initio data, taking into account radial coupling between the X 2Σ + and A 2Π 1 2 states.

J dependence of predissociation rates and hyperfine constants as measured by high resolution laser spectroscopy

Highly excited rotational states in electronically excited bromine molecules are measured with full hyperfine resolution, by means of sub-Doppler polarization spectroscopy. The dependence of molecular parameters such as predissociation rates and hyperfine coupling constants on the rotational quantum number is extracted, and found to be proportional to the rotational energy J(J+1), extending previous data to much higher rotational states. By fitting a large set of hyperfine resolved rotational spectra from one vibrational band to a single set of parameters, very small dependencies may be extracted. The predissociation rates measured here in the frequency domain are compared to previous measurements in the time domain, and found to be smaller by more than an order of magnitude. We show that extracting decay times from lines of unresolved hyperfine structure may be misleading, and attribute this discrepancy to dephasing due to interferences between hyperfine components.

A b i n i t i o study of the three lowest states X 2∑+, 2Π1/2, 2Π3/2, and B 2∑+ of the HeNe+ ion: Potential energy curves, Λ doubling, and predissociation rates of the rotational levels in the 2Π1/2 (v=0) state

Multireference configuration interaction calculations are performed for the X 2∑+, 2Π, and B 2∑+ states in HeNe+ from 2.2 to 100 bohr. The various perturbations of the Born–Oppenheimer states are evaluated: the spin–orbit interaction is calculated up to second order by employing the Breit–Pauli Hamiltonian. The resulting potential curves for the A 2Π1/2 state are compared with the experimental data of Dabrowski and Herzberg (Ref. 1), the curve of the hereto undetected A 2Π3/2 is predicted. The Λ splitting of the A1 2Π3/2 and the A2 2Π1/2 states due to coupling of the electronic motion with the rotation of the nuclear framework is computed by employing a basis of vibronic eigenfunctions. The calculated parameters of p0=−1.407 cm−1 for A2 2Π1/2 agree well with the corresponding data of p0=−1.417 or −1.427 cm−1 derived from measurements (Refs. 1 and 2). The predissociation rates of the rotational levels in the A2 2Π1/2 (v=0) state are also calculated on the basis of the computed ab initio data. All results are compared with the experimental data of Carrington (Ref. 2). The mechanism for the predissociation of the A 2Π1/2 state (in the absence of a curve crossing) is elucidated.

On the measurement of (predissociation) rates in the frequency and time domains

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Picosecond real-time studies of mode-specific vibrational predissociation

The vibrational predissociation of several van der Waals complexes of t-stilbene has been studied by directly measuring, in real time, the fluorescence intensity from the initial reactant state and from the individual product states formed in the dissociation process after exciting single vibrational levels of the complex. With the aid of a kinetic model involving sequential processes, the individual rates for intramolecular vibrational redistribution and vibrational predissociation in the overall dissociation process are resolved and distinguished in several cases. In the stilbene–He complex, the dissociation is significantly faster from low energy out-of-plane modes than it is from a higher energy in-plane mode.

State-specific predissociation of N2(B 3Πg) measured by laser-induced fluorescence on a molecular beam

Using sub-Doppler laser induced fluorescence spectroscopy on a beam of metastable nitrogen N2(A 3∑+u), the predissociation of the N2(B 3Πg) state was studied quantitatively and under collision-free conditions. Predissociation rate constants γ in the range 0.1–200×106/s were measured (a) from the decreased fluorescence quantum yield and (b) from the increased linewidth. In method (a), coherent saturation effects were quantitatively accounted for. For the 3Πe0,3Πe,f1, and 3Πe,f2 levels, γ is generally high and independent of the molecular rotation in the range J=1–8, while for 3Πf0 levels γ is smaller and increases proportionally to J(J+1). This pattern identifies the N2(A′ 5∑+g) state uniquely as being responsible for the perturbation of N2(B). A pronounced dependence of γ on the vibrational level of N2(B) was also found in the range v′=13–18. It correlates quantitatively with calculated bound–free Franck–Condon factors between the B and A′ states. For the latter, a recent ab initio potential curve was used, with a slight (0.02 Å) radial adjustment. The B–A′ electronic coupling matrix element is on the order of 2 cm−1, much smaller than expected for a spin–orbit mechanism. This can be understood on the basis of the CI composition of the interacting states as calculated ab initio.

The linear and T-shaped isomers of C 2H 2HCN: Vibrational dynamics from infrared spectroscopy and ab initio theory

The optothermal detection technique has been used to obtain infrared spectra of the HCN-acetylene complex. In addition to the T-shaped structure observed previously by microwave spectroscopy, a linear isomer also has been discovered and characterized.For both isomers the acetylenic and hydrogen cyanide CH stretch vibrational bands have been recorded. The results clearly show that the predissociation lifetimes of these isomers depend strongly upon whether or not the CH vibration excited is involved directly in the formation of the hydrogen bond. Ab initio calculations also have been performed for both isomers at the 6- 31G∗∗/MP2 level. Comparisons between these calculations and the experimental results suggest that, for this hydrogen bonded system, an ab initio multi-dimensional potential surface would be quite realistic. Of particular importance to obtaining a better understanding of the mode dependence of the vibrational predissociation rates is the fact that the calculations reproduce the spectroscopic data which is most sensitive to the coupling between the intramolecular and intermolecular vibrational coordinates.

Vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 clusters

The first excited electronic state (S1) vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 van der Waals (vdW) clusters have been studied using molecular jet and time resolved emission spectroscopic techniques. The rates of intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) as functions of vibrational energy are reported for both clusters. For vibrational energy in excess of the cluster binding energy, both clusters are observed to dissociate. The dispersed emission spectra of these clusters demonstrate that aniline(Ar)1 dissociates to all energetically accessible bare molecule states and that aniline(CH4)1 dissociates selectively to only the bare molecule vibrationless state. The emission kinetics show that in the aniline(Ar)1 case, the initially excited states have nanosecond lifetimes, and intermediate cluster states have very short lifetimes. In contrast, the initially excited aniline(CH4)1 states and other intermediate vibrationally excited cluster states are very short lived (<100 ps), and the intermediate cluster 00 state is observed. These results can be understood semiquantitatively in terms of an overall serial IVR/VP mechanism which consists of the following: (1) the rates of chromophore to vdW mode IVR are given by Fermi’s golden rule, and the density of vdW vibrational states is the most important factor in determining the relative [aniline(Ar)1 vs aniline(CH4)1] rates of IVR; (2) IVR among the vdW modes is rapid; and (3) VP rates can be calculated by a restricted vdW mode phase space Rice–Ramsberger–Kassel–Marcus theory. Since the density of vdW states is three orders of magnitude greater for aniline(CH4)1 than aniline(Ar)1 at 700 cm−1, the model predicts that IVR is slow and rate limiting in aniline(Ar)1, whereas VP is slow and rate limiting in aniline(CH4)1. The agreement of these predictions with the experimental results is very good and is discussed in detail.

Slit jet infrared spectroscopy of NeHF complexes: Internal rotor and J-dependent predissociation dynamics

Direct absorption tunable difference frequency IR spectroscopy in a slit jet supersonic expansion has been used to observe complexes of Ne with HF for the first time. Spectra of both the weak HF stretch fundamental (1000)←(0000) and the 10–20 fold more intense bend and stretch combination band (111e,f0)←(0000) transitions are observed, and illustrate several interesting dynamical features. The large ratio of combination band to fundamental intensity is evidence for a highly isotropic potential with respect to HF rotation. The HF bend vibration is thus better thought of as nearly free internal rotor motion with a nearly good space fixed quantum number, jHF =1. Weak anisotropy in the potential permits the jHF=1 (Πebend) levels to relax intramolecularly to jHF =0 (Σ) levels, leading to predissociative line broadening in the sub-Doppler slit jet spectra. This observed dissociation of NeHF with 44 cm−1 of internal excitation provides a rigorous upper limit to the binding energy. The Πfbend levels, on the other hand, have no lower energy internal rotor states of the correct parity to relax into, and thus the (111f0) ←(0000) spectra exhibit linewidths limited by the apparatus resolution. The internal relaxation of the jHF=1 (Πebend) levels results from Coriolis mixing with the Σbend states (1200) by overall rotation of the NeHF complex, and thus the lifetimes depend dramatically on J. The J-dependent predissociation rates observed are in remarkably quantitative agreement with quantum scattering calculations on the CEPA ab initio surface of ONeil and co-workers.