L. Oudejans

Pendular state spectroscopy of an asymmetric top: Parallel and perpendicular bands of acetylene-HF

We report here a comprehensive theoretical and experimental study of the pendular state spectroscopy of a near-prolate asymmetric top molecule and consider the specific case of the acetylene-HF binary complex in a static electric field. The experiments were carried out using the optothermal detection method and the spectra were recorded in an electric field of sufficient magnitude to give rise to substantial orientation in a number of states. The calculated and experimental spectra are compared for different vibrational band types and polarization configurations. In this way we have identified the states that give the best orientation and thus are well suited to photodissociation studies. The hybridized orientational probability distributions are presented for the relevant states.

Infrared spectroscopy and ab initio potential energy surface for Ne-C2H2 and Ne-C2HD complexes

The rotationally resolved spectra of Ne–C2H2 and Ne–C2HD were measured in the region of the asymmetric C–H stretch (ν3) band of the acetylene monomer. The transitions in the Ne–C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne–C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a “doorway” state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne–acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne–C2H2 and Ne–C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the ν3 band. These complexes may be considered as nearly free internal rotors. For Ne–C2H2 the results obtained from the ...

State-to-state vibrational predissociation dynamics of the acetylene-HF complex

The vibrational predissociation of the acetylene-HF complex has been studied by probing the final state distributions of the photofragments, including the intermolecular scalar correlations. Results are presented for excitation of both the asymmetric C–H stretching vibration of the acetylene and the H–F stretch. In the latter case, the primary dissociation channel is V–R, resulting in highly rotationally excited HF produced in coincidence with ground vibrational state acetylene. Excitation of the asymmetric C–H stretch results in intramolecular V–V energy transfer to the C–C stretch excited state of the acetylene. The dissociation energy of the complex is determined to be 1088 cm−1±2 cm−1.

Vibrational predissociation in the HCl dimer.

We present results of a combined theoretical and experimental study on the vibrational predissociation of the HCl dimer. On the theoretical side, photodissociation linewidths and product-state distributions for monomer stretch excited states with total angular momentum J=0 were computed, using the Fermi golden rule approximation. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the intermolecular stretch and geared bend modes, for both even and odd permutation symmetry. Line strengths for the transitions from the J=1, K=0 ground state to excited states with J=0 were computed using quasibound states. On the experimental side, the photofragment angular distribution method was employed to obtain complete final-state distributions for the monomer stretch excited states. Three different transitions were probed, all starting from the lower tunneling component of the ground state: the (R)Q(0)(1) transition for excitation of the acceptor stretch and the (Q)R(0)(0) transition and unresolved (R)Q(0) branch for the donor stretch excitation. We find that, in contrast to the HF dimer, the excited-state alignment of the HCl dimer, resulting from excitation using a polarized laser beam, is completely lost on the time scale of the dissociation. The agreement between theory and experiment for the product-state distributions and line strengths is reasonable. The computed lifetimes are 1-2 orders of magnitude too small, which is attributed to a deficiency in the potential energy surface.

High resolution infrared spectroscopy and ab initio calculations of HCN–H2/D2 binary complexes

High-resolution infrared laser spectroscopy has been used to study HCN–H2 and HCN–D2 complexes in the gas phase. The experimental results are compared with ab initio calculations that are also reported here. The latter calculations reveal two prominent minima on the potential surface, one corresponding to a “T-shaped” complex with the H2 at the hydrogen end of the HCN and the other a “linear” complex with the H2 H-bonded to the nitrogen. The latter minimum is the global minimum on the surface, in agreement with the fact that this structure is observed experimentally for both o-H2 and p-D2.

The state-to-state predissociation dynamics of OC–HF upon HF stretch excitation

Photofragment angular and state distributions have been measured following the vibrational predissociation of the OC–HF complex. An F-center laser is used to pump the fundamental H–F stretching vibration of the complex and a second F-center laser is used to probe the rotational states of the HF fragment as a function of recoil angle. The complex dissociates via two different sets of channels, one that produces vCO=1, JHF=6,5,4 (intermolecular V–V transfer) and the other vCO=0, JHF=11 (V–R transfer). Analysis of the data gives correlated final state distributions, as well as an accurate value for the dissociation energy (D0) of the complex, namely 732±2 cm−1.

Photodissociation of cyclic HF complexes: Pentamer through heptamer

Photofragment angular and final rotational state distributions have been measured, corresponding to the vibrational predissociation of HF polymers [(HF)N N=5–7]. The present experiments confirm the theoretical prediction that, for clusters in this size range, the only open dissociation channel corresponds to the “evaporation” of a single HF monomer. The pump–probe experiments reported here indicate that the resulting HF monomer is produced with only modest rotational excitation. Rotational temperatures of 180 and 140 K have been determined for the evaporated HF monomer following dissociation of the HF pentamer and hexamer, respectively. Energy conservation in these photodissociation studies provides upper limits for the dissociation energies (D0), namely, 2941 cm−1 and 2854 cm−1 for the pentamer and hexamer, respectively.

Intermolecular V–V energy transfer in the photodissociation of CO2–HF(v=1)

Photofragment final state distributions have been measured for the vibrational predissociation of CO2–HF corresponding to excitation of the H–F stretching vibration. The method used in these studies combines photofragment translational spectroscopy, pendular state orientation methods, and laser probing to provide distributions that include the interfragment state correlations. The results clearly show that the dominant dissociation channel involves intermolecular V–V energy transfer corresponding primarily to excitation of the asymmetric stretching vibration of the CO2 fragment. The dissociation energy of the complex has also been determined to be 672±4 cm−1.

The infrared spectroscopy and dynamics of OCO–HCl and SCO–HCl: An example of mode specific intermolecular energy transfer

Optothermal near infrared laser spectroscopy has been used to study the OCO–HCl and SCO–HCl complexes by exciting the H–Cl stretch using an F‐center laser. In both cases, the two isotopic forms associated with H35Cl and H37Cl have been observed. All of the observed spectra are consistent with and analyzed in terms of a linear structure. Vibrational predissociation is observed to be abnormally fast in the OCO–HCl complex, considering the rather weak coupling between the intermolecular degrees of freedom and the H–Cl stretch suggested by the small vibrational frequency shift associated with complex formation. Comparisons are made between the two systems studied here that indicate that this anomalous rate is due to a near resonance between the energy available to the fragments after the photodissociation of the complex and the asymmetric stretch of the CO2 fragment.

Extreme metastability in Ar–HCl (v=1) and Ar–DF (v=1)

High resolution optothermal spectra have been obtained for the Ar–HCl and Ar–DF complexes which conclusively show that they do not dissociate on H/D–X vibrational excitation. The observed linewidths place a lower limit on the lifetimes of 500 ns, while time‐of‐flight considerations indicate that the lifetimes are in excess of 0.4 ms. Stark spectra are also reported, which result in the determination of the (v=1) excited state dipole moments of these complexes.