Anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant

Predissociation dynamics of D2 revealed by variation in fragment anisotropy parameters along the Fano profile

We conducted a study on the variations of the fragment anisotropy parameters (β) along the Fano profiles for the predissociation of the D2 molecule. These variations, known as β profiles, were measured for the D(2l) fragments from the predissociation of the 4pπD'Πu1υ'=1 and 4pσB″Σu+1υ'=2 states. The measured β profiles show significant asymmetry and broader linewidths compared to the corresponding Fano profiles. By fitting the β profiles, we were able to determine the fragment anisotropy parameters associated with the resonance state, continuum state, and the interference effect between them. Additionally, we determined the ratios of the absorption cross sections between the unperturbed and perturbed continuum states interacting with the resonance states although these ratios were found to be very small. Furthermore, we derived approximate formulas to calculate the parameters characterizing the β profile. Despite the linewidths of the four Fano profiles being narrower than our instrumental resolution, we were still able to determine the product of the linewidth with the Fano q parameters. These findings demonstrate the utility of the β profile as an effective tool for studying the predissociation dynamics in diatomic molecules.

Spectra and Predissociation Dynamics of the 4pσB''1∑u+ and 4pπD' 1Πu States of D2

The spectra and predissociation dynamics of the Rydberg states 4pσB″1∑u+(υ'=2) and 4pπD'1Πu±(υ'=1) of the D2 molecule have been studied by a combination of XUV laser pump, UV laser probe, and velocity map imaging methods. The D(2l) fragment yield spectrum and the D2 excitation spectrum have been measured. Although the predissociation rates were found to be small and comparable to the radiative decay rates, the D(2l) fragments were observed for all the Rydberg states. The measured D(2l) fragment angular distributions are in good agreement with a reported model that has an analytical formula for anisotropy parameters. The results demonstrated that the 4pσB″1∑u+ and 4pπD'1Π u+ states dissociate via direct and indirect coupling with an 1∑u+ state, respectively. For predissociation of the 4pσB″1∑u+ state, the experimental data showed that D(1s) + D(2s) is the main channel, supporting previous theoretical predictions that the predissociation occurs by coupling to the vibrational continuum of the 3pσB'1∑u+ state.

Predissociation dynamics in the 3pπD(1)Πu (±)υ=3 and 4pσB(″) (1)Σu (+)υ=1 states of H2 revealed by product branching ratios and fragment angular distributions

The predissociation dynamics of H2+XUV→H2 (*)→H(1s)+H(2s,2p) has been studied by measuring the fragment branching ratios between the H(2s) and H(2p) states and the fragment angular distributions using the XUV (extreme ultraviolet) laser pump and UV(ultraviolet) laser probe method. The fragment angular distributions for the predissociation of the 3pπD(1)Πu (+)υ=3 state show parallel transitions, demonstrating that the main components of the dissociating state have (1)Σu (+) symmetry. The measured fragment branching ratios, H(2s)/(H(2s) + H(2p)), for the transitions R(0), R(1), and P(2) in 3pπD(1)Πu (+)υ=3←X(1)Σg (+)υ(″)=0 are in good agreement with one of the previous theoretical predictions. The predissociations of the 3pπD(1)Πu (-)(υ=3) state arising from the Q(1), Q(2), and Q(3) lines have also been observed. The angular distributions and the state distributions of the excited fragments (all found from the H(2p) state) illustrate that the dissociating states for the Q lines have the expected Πu (-) symmetry. The predissociation dynamics of the transition 4pσB(″1)Σu (+)υ=1←X(1)Σg (+)υ(″)=0 was also studied. Their fragment angular distributions show the expected parallel transitions, and most of the fragments are in the H(2s) states. The Beutler-Fano profiles and the associated spectroscopic parameters for the predissociations have also been obtained by measuring the fragment yield of H(2s, 2p) as a function of excitation photon energies.

Rotational and angular distributions of NO products from NO-Rg (Rg = He, Ne, Ar) complex photodissociation

We present the results of an investigation into the rotational and angular distributions of the NO à state fragment following photodissociation of the NO-He, NO-Ne, and NO-Ar van der Waals complexes excited via the à ← X̃ transition. For each complex, the dissociation is probed for several values of Ea, the available energy above the dissociation threshold. For NO-He, the Ea values probed were 59, 172, and 273 cm(-1); for NO-Ne they were 50 and 166 cm(-1); and for NO-Ar they were 44, 94, 194, and 423 cm(-1). The NO à state rotational distributions arising from NO-He are cold, with most products in low angular momentum states. NO-Ne leads to broader NO rotational distributions but they do not extend to the maximum possible given the energy available. In the case of NO-Ar, the distributions extend to the maximum allowed at that energy and show the unusual shapes associated with the rotational rainbow effect reported in previous studies. This is the only complex for which a rotational rainbow effect is observed at the chosen Ea values. Product angular distributions have also been measured for the NO à photodissociation product for the three complexes. NO-He produces nearly isotropic fragments, but the anisotropy parameter, β, for NO-Ne and NO-Ar photofragments shows a surprising change in sign from negative to positive as Ea increases within the unstructured excitation profile. Franck-Condon selection of a broader distribution of geometries including more linear geometries at lower excitation energies and more T-shaped geometries at higher energies can account for the changing recoil anisotropy. Two-dimensional wavepacket calculations are reported to model the rotational state distributions and the bound-continuum absorption spectra.

Probing Rotational Motion in 4-tert-Butylcatechol through H Atom Photofragmentation: Deviations from Axial Recoil

The time-resolved photofragmentation dynamics of 4-tert-butylcatechol were studied following one photon excitation to the S1 (1(1)ππ*) state with ultraviolet radiation in the range 260 ≤ λ ≤ 286 nm. The preparation of an aligned molecular ensemble via photoexcitation leads to anisotropy in the H atom photofragments. These H atoms originate from the decay of the S1 state through coupling onto the S2 ((1)πσ*) state, which is dissociative along the nonintramolecular hydrogen bonded "free" O-H bond. The degree of anisotropy of these photogenerated H atoms decreases with increasing pump-probe time delay. This is attributed to rotational dephasing of the initially aligned molecular ensemble. The measured dephasing occurs on a time scale akin to the appearance time of these H atoms, which likely places an intrinsic lower bound on the dephasing lifetime. The present work demonstrates how a careful balance between the appearance time of the H atoms, determined by the S1 lifetime, and the rotational dephasing in 4-tert-butylcatechol provides an opportune window to probe rotational motion in real time.

NO dissociation through ns, np, and nf Rydberg states: angular distributions

Velocity-mapped imaging and theoretical calculations have been used to study the angular distribution of the products of NO predissociation following its excitation to the 11s, 10p, 11p, and 9f Rydberg levels based on the NO(+) (X (2)Σ(+)) core. The Rydberg states were reached from the NO (A (2)Σ(+), v = 0, N = 2, J = 1.5) level prepared with strong alignment by excitation with linear polarization from NO (X (2)Π, v = 0, N = 1, J = 0.5). Ion dip spectra of the Rydberg states were recorded along with velocity-mapped images at the major peaks. The results are compared to calculations based on a previous theoretical approach modified to include transitions to states of Hund's case (d) coupling. The reasonable agreement shows the predictive value of the theory. The theory has also been used to reassess and explain previous results and to understand variations in the rate of photodissociation with components of the 10p and 11p Rydberg states.

Experimental and theoretical investigation of correlated fine structure branching ratios arising from state-selected predissociation of BrO (A2Π3/2)

We present results for the v'-dependent predissociation dynamics of the BrO (A(2)Π3/2) state using velocity map ion imaging. Correlated fine structure branching ratios, Br((2)P(J)) + O((3)P(J)), have been measured for v' = 5-16 states. The experimental branching ratios are non-statistical and strongly dependent on the initial vibronic state. The current measurements represent an extensive dataset containing rich information about the predissociation dynamics of this system and should provide a stringent test for modern theory. New high level ab initio excited state potentials are presented and have been optimized using experimental v'-dependent predissociation lifetimes and calculated coupling constants. Comparisons between the experimental branching ratios and the predictions based on diabatic and adiabatic limiting models are presented. We find that the adiabatic model is most consistent with the observed trends in the correlated branching ratios, in contrast to previous studies on the related ClO system.

Time-sliced velocity-mapped imaging studies of the predissociation of single ro-vibronic energy levels of N2 in the extreme ultraviolet region using vacuum ultraviolet photoionization

The predissociation of N(2) from the rotational levels in the o(1)∏(u) (v(') = 2) and b(') (1)Σ(u) (v(') = 8) bands has been studied in the wavenumber (or energy) range from 109 350 cm(-1) (13.5577 eV) to 109 580 cm(-1) (13.5862 eV) by time-sliced velocity-mapped imaging technique with VUV photoionization detection of the fragments. These levels were excited from the ground state of N(2) (X(1)Σ(g) (+), v(") = 0) levels using an unfocused vacuum ultraviolet (VUV) laser via a one-photon process. The same VUV laser is used to ionize the metastable N ((2)D(o)) produced from the predissociation process and the time-sliced velocity-mapped imaging technique is used to determine their velocity and angular distributions. Two different theoretical methods developed, respectively, by Kim et al. [J. Chem. Phys. 125, 133316 (2006) and Zande [J. Chem. Phys. 107, 9447 (1997)] were used to calculate the anisotropic parameters for the predissociation to the channel N((4)S(o)) + N((2)D(o)) to compare with the observed value for each of the rotational levels. Very good agreement with the experimental results was obtained for both methods. Possible predissociation mechanisms were predicted from the measurements and calculations.

Photofragment angular momentum distribution beyond the axial recoil approximation: Predissociation

We present the quantum mechanical expressions for the angular momentum distribution of the photofragments produced in slow predissociation. The paper is based on our recent theoretical treatment [J. Chem. Phys. 123, 034307 (2005)] of the recoil angle dependence of the photofragment multipole moments which explicitly treat the role of molecular axis rotation on the electronic angular momentum polarization of the fragments. The electronic wave function of the molecule was used in the adiabatic body frame representation. The rigorous expressions for the fragment state multipoles which have been explicitly derived from the scattering wave function formalism have been used for the case of slow predissociation where a molecule lives in the excited quasibound state much longer than a rotation period. Possible radial nonadiabatic interactions were taken into consideration. The optical excitation of a single rotational branch and the broadband incoherent excitation of all possible rotational branches have been analyzed in detail. The angular momentum polarization of the photofragments has been treated in the high-J limit. The polarization of the photofragment angular momenta predicted by the theory depends on photodissociation mechanism and can in many cases be significant.