D. David

Mode‐selective bond fission: Comparison between the photodissociation of HOD (0,0,1) and HOD (1,0,0)

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H or O–D stretching vibrational energy is described. Stimulated Raman excitation and coherent anti‐Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) (O–H stretch) or (1,0,0) (O–D stretch) vibrationally excited HOD. The OD and OH fragments are detected by laser‐induced fluorescence. In the photodissociation of HOD (0,0,1), the yield of both fragments is enhanced [relative to HOD (0,0,0)], but the yield of OD is increased 2.5±0.5 times more than that of OH. In the photodissociation of HOD (1,0,0), no enhancement of the yield of the fragments is obtained. Our results show that even the very lowest possible level of vibrational excitation can be ‘‘leveraged’’ to effect selective bond breaking. Also, these results demonstrate that bond cleavage does not necessarily occur on the weakened bond and they agree with theoretical calculations indicating that the yield of OD and O...

Direct observation of preferential bond fission by excitation of a vibrational fundamental: Photodissociation of HOD (0,0,1)

The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H stretching vibrational energy is described. Stimulated Raman excitation and coherent anti‐Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) vibrationally excited HOD. The OD and OH fragments are detected by laser induced fluorescence. The photodissociation of the HOD (0,0,1) molecules yields at least three times more OD than OH.

State‐to‐state photodissociation of the fundamental symmetric stretch vibration of water prepared by stimulated Raman excitation

The state‐to‐state photodissociation at 193 nm of the fundamental symmetric stretch vibration of water, H2O (1,0,0), is studied. Stimulated Raman excitation and coherent anti‐Stokes Raman scattering are used to prepare and detect, respectively, particular rotational states of H2O (1,0,0). Laser induced fluorescence is used for monitoring the OH species which are formed from particularly selected rotational states of the H2O (1,0,0) and also from photodissociation of all occupied rotational states of the ground vibrational state, H2O (0,0,0), at room temperature. The cross section for photodissociation from a particular rotation of H2O (1,0,0) at 193 nm is found to be ∼550 times greater than that for H2O (0,0,0). The formation of the OH product in different rotational, Λ‐doublet and spin–orbit states is analyzed for the photodissociation of H2O (0,0,0) and for the photodissociation of the 101, 110+111, 212+211, and 303 rotational states of H2O (1,0,0). The rotational distribution of the OH resulting from p...

Control of fragment alignment via photodissociation from different types of parent rotation

The alignment of the OH fragment following dissociation of H2O (1,0,0) at 193 nm has been measured as a function of initially prepared parent rotations. The degree of alignment is almost perfect for an in‐plane rotation, but is reduced markedly for a mixture of in‐ and out‐of‐plane rotations.

Rotationally mediated vector correlations in the photodissociation of H2O (1,0,0)

Abstract The vector correlations following the photodissociation of initially prepared rotational states of the fundamental symmetric stretch of water, H 2 O (1,0,0), are studied in detail. The rovibrationally excited water molecules are prepared by stimulated Raman excitation and photodissociated at 193 nm via the first electronically excited state. The OH fragments are probed by both broadband and sub-Doppler polarization spectroscopy. It is shown that the degree of rotational alignment (the μ- J correlation) obtained when the parent molecule is prepared in an in-plane rotational state (3 03 ) is a factor of two higher than in a state containing a mixture of in-plane and out-of-plane rotations (3 21 + 3 22 + 4 14 ). Also, the μ-ν, ν- J and μ-ν- J correlations following the photodissociation of the 3 03 state are close to the limiting values expected for an idealized orientation (with minimal influence of the parent rotation) where the transition dipole moment (μ) of the parent is parallel to the fragment angular momentum ( J ) and perpendicular to its velocity (ν). In addition, from the degree of misalignment due to the out-of-plane rotation, an estimate of a lifetime of 40 fs of H 2 O in the first electronically excited state is obtained. These results demonstrate that experiments which prepare the parent molecule in a particular rotational state, before a second laser dissociates it, provide a powerful means for understanding the directional characteristics of dissociation processes.

Coherent Anti-Stokes Raman Spectroscopy of the Stretching Vibrations of the Water Isotopomers:

Coherent anti-Stokes Raman spectroscopy (CARS) has been employed to study the v1 band of H2O, v1 band of DzO, and v1 and v3 bands of HOD in the gas phase at room temperature and at moderate resolution. These molecules are characterized by a complicated spectrum which is not completely rotationally resolved. A method for the deconvolution of incompletely resolved CARS spectra has been utilized, in which a Boltzmann population distribution is assumed. The calculated spectrum nearly exactly reproduces the observed spectrum and allows assignment of the rotational transitions which are observed in the spectra. All the transitions that appear in these spectra belong to the isotropic Q branches. The CARS spectra of D2O and HOD are studied here for the first time.

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.

Mode-Selective Bond Fission in Fundamental Stretchings of Hod

Stimulated Raman excitation and coherent anti-Stokes Raman scattering prepare and detect, respectively, HOD molecules withonequantum of vibrational excitation in the O-H or OD stretch vibrations. This selective vibrational excitation prepares densities of the excited molecules in particular rotational levels that are sufficient for molecular reaction dynamics studies. An ArF excimer laser at 193 nm promotes vibrationally excited molecules to the first electronic surface A(1B 1) where they dissociate to produce OH + D or OD + H fragments. The photodissociation products OD and OH are detected via laser induced fluorescence. The photodissociation cross section of HOD (0,0,1) vibrationally excited molecules with one quantum of vibration in the O-H stretch is greatly enhanced over that for HOD (0,0,0). The dissociation of this vibrationally excited molecule is also very selective, the OD + H yield being 2.5 ± 0.5 times greater than the OH + D yield. In the photodissociation of the O-D stretch, excited HOD (1,0,0), no enhancement of the yield of the fragments is obtained. These results demonstrate that bond cleavage does not necessarily occur on the weakened bond, and they agree with theoretical calculations indicating that the yield of OD and OH fragments depends on the Franck-Condon overlap of the ground state vibrational wavefunction with the continuum wavefunction on the repulsive surface of the upper state. Also, our results show that even the very lowest possible level of vibrational excitation can be “leveraged” to effect selective bond breaking.