Yuval Ganot

Acetylenic C–H and methyl C–D bond fission in photodissociation of vibrationally excited propyne-d3

Vibrational preexcitation of a state containing three quanta of C–H stretch (3ν1) results in C–H acetylenic and C–D methyl bond rupture in the ∼243.1 nm photolysis of CD3C≡CH, in contrast to previous observations of the almost isoenergetic 193 nm photodissociation of propynes. The C–D bond fission is the dominant pathway with a D/H branching ratio of 2.0±0.5 at a combined energy of ∼50 830 cm−1. The average translational energies of D and H atoms are nearly identical, although the C–H acetylenic and C–D methyl bond energies differ quite extensively, pointing to different dynamics on the involved potential energy surfaces.

Combination bands versus overtone stretch excitation and rotational effects in vibrationally mediated photodissociation of acetylene

Rovibrational excitation combined with promotion of C2H2 molecules to the excited electronic trans-bent states A 1Au/B 1Bu and photofragment ionization are used to generate action spectra, H Doppler profiles, and time-of-flight mass spectra. Rovibrational states of C2H2 in the 15 480–15 723 cm−1 region are photodissociated by 243.135 nm photons that subsequently tag the H fragments. The H photofragment yield is greatly enhanced upon rovibrational excitation. In the action spectra, the intensities of the combination bands that involve high stretch and low bend excitation, (1410020) and (2031100), are close to that of the fourth overtone of the C–H stretch, (2030000), while in the absorption spectra the intensities of the combination bands are much weaker. In addition, the effect of rotation on fragment yield and dissociation channels is demonstrated. Several pairs of rotational transitions (sharing similar J′) stand out from the rotational band contour and the R(13) line of (2030000) shows an anomalously ...

Disclosing rovibrational couplings and overlaps from irregularities in action spectra: Photodissociation of the 4νCH rovibrational manifold of C2H2

Absorption and free-jet action spectroscopy of states in the region of four quanta of C–H stretching of acetylene (C2H2) around 12 676 cm−1 were studied. Rovibrational excitation combined with 243.135 nm promotion of C2H2 molecules to the upper electronic trans-bent states, A 1Au/B 1Bu, and H photofragment ionization generated action spectra measuring the H yield as a function of the excitation wavelength. The findings show that the (1030000) IR bright state of the third C–H stretch overtone has a smaller photodissociation cross section than the (1214000) combination band containing trans-bend mode excitation and lying in its vicinity, due to a favorable Franck–Condon factor for the latter. Certain line pairs accessing similar J′ levels of the (1030000) state show anomalous intensities in the action spectrum, with P transitions excessively enhanced over R transitions. This implies enormous dissociation efficacy as a consequence of Coriolis-type local resonances with nearby states of another band and due ...

Enhanced stimulated Raman scattering in temperature controlled liquid water

The dependence of single pass stimulated Raman scattering (SRS) in liquid water on temperature was investigated. Thermal self-defocusing and competition with backward stimulated Brillouin scattering were found to be the major limiting factors for forward SRS (FSRS) generation. Experiments were performed to show that water cooling reduces these factors, resulting in significant enhancement of the FSRS and leading to a maximum conversion efficiency of 35% for pump energy of 120 mJ/pulse and for water at a temperature of 4 °C. Computer simulations of the involved waves resulted in FSRS efficiencies agreeing well with the experimental ones.

H and D release in ∼243.1 nm photolysis of vibrationally excited 3ν1, 4ν1, and 4νCD overtones of propyne-d3

The photofragmentation of propyne-d3, D3C–C≡C–H, following ∼243.1 nm photodissociation of rovibrationally excited molecules promoted to the second (3ν1) and third (4ν1) acetylenic C-H overtone and to the third (4νCD) methyl overtone has been investigated. The resulting H and D photoproducts were detected via (2+1) resonantly enhanced multiphoton ionization. The measured room-temperature photoacoustic and jet-cooled action spectra allowed derivation of the molecular parameters of the C-H overtones and the Doppler profiles revealed the translational energies associated with the H(D) photofragments and the H to D branching ratios. Propensities toward the latter were encountered, while the translational energy disposal in both photofragments was essentially identical for a given preexcitation. This behavior agrees with that found for the almost isoenergetic 193.3 nm photolysis of propyne [Qadiri et al., J. Chem. Phys. 119, 12842 (2003)], but contradicts previous findings. The bond fission of C-H and C-D is pr...

Evidence for new bands in the 3ν1 and 4ν1 regions of propyne

Vibrationally mediated photodissociation and room-temperature photoacoustic (PA) spectroscopy have been used for obtaining action (monitoring the yield of H photofragments) and absorption spectra of the second (3ν1) and third (4ν1) C–H acetylenic stretches overtone regions in propyne. The band contours appearing in these regions seem mostly regular even though they are perturbed, as expressed by the origin shifts in different K components, splitting of the K structure, and splitting due to resonances between neighboring states. Symmetric rotor simulations of the band contours of the PA and action spectra allowed extraction of the molecular parameters and rough estimates for the homogeneous broadening arising from energy flow to the bath vibrational states. We particularly benefited from the reduced congestion in the jet-cooled action spectra and their simulations, which enabled observation of yet unknown features in the vicinity of the 3ν1 and 4ν1 states. Particularly, the emergence of the new state in th...

Differing reactivities in the predissociation of acetylene isotopomers pre-excited with three CH stretching quanta

Abstract Selective photolysis of a mixture of acetylene isotopomers prepared in rovibrational states lying in the energy window of the second overtone of CH stretches was performed. Tunable near infrared (NIR) photons prepared the isotopomers in initial states and ultraviolet photons both promoted them to the excited electronic trans-bent A 1 A u state and tagged the H(D) photofragments via (2+1) resonantly enhanced multiphoton ionization. Photoacoustic spectroscopy monitored the initial state preparation and action spectroscopy simultaneously tagged the H(D) photoproducts yield as a function of the wavelength of the exciting NIR laser. The measurements and analysis indicate differing dissociation efficacies in C2H2 and C2HD revealing enhanced CH bond cleavage in the former and hindered CH(D) bond fission in the latter. The enhancement is a result of coincident resonances with the upper state leading to sharp structures in the photodissociation cross-section of C2H2.

Determining the vibrational pattern via overtone cold spectra: C-H methyl stretches of propyne.

Vibrationally mediated photodissociation and photoacoustic (PA) spectroscopy were employed for studying the intramolecular dynamics of propyne initially excited to the first through fourth overtone of methyl C-H stretching modes. Room-temperature PA and jet-cooled action spectra, monitoring the absorption of the parent and the yield of the ensuing H photofragments, respectively, were obtained. The PA spectra exhibit mainly broad features, while the action spectra, due to inhomogeneous structure reduction, expose multiple peaks of recognizable shapes in the differing overtone manifolds. Symmetric rotor simulations of the band contours of the action spectra allowed retrieving of band origins and linewidths. The linewidths of the bands in each manifold enabled estimates for energy redistribution times out of the corresponding states to the bath states, the times ranging from 18+/-6 ps for two quanta of C-H excitation to subpicosecond for five quanta. The data were also analyzed in terms of a normal-mode model and a joint local-/normal-mode model. These models enabled determination of harmonic frequencies, anharmonicities, and interaction parameters reproducing the observed data in all monitored regions and provided spectral assignments. The measured Doppler profiles were well fitted by Gaussians with widths suggesting low average translational energies for the released H photofragments. These low energies and their similarities to those for dissociation of propyne isotopomers preexcited to acetylenic C-H stretches were ascribed to an indirect dissociation process occurring after internal conversion to the ground electronic state and isomerization to allene.

Non-adiabatic dissociation of rovibrationally excited acetylene

The photofragmentation dynamics of acetylene, C2H2, was explored via vibrationally mediated photodissociation. Direct near infrared (NIR) excitation efficiently prepared rovibrational states in the region of three C–H stretch quanta (∼9640 cm−1), subsequently ∼243.1 nm UV1 photons promoted the pre-excited C2H2 molecules to the A 1Au state and dissociated them and finally the ensuing H atoms were probed by UV2 photons via (2+1) resonantly enhanced multiphoton ionization. UV dependent action spectra, monitoring the H photofragment yield vs. the UV1 dissociating laser wavelength displayed sharp peaks depending on the combined energy and the initially excited rovibrational state. These spectra indicate that the rovibrational transitions belonging to the (1112000) combination band, containing trans-bend mode excitation, are excessively enhanced over those of the (0030000) state with three C–H stretch quanta, due to favorable Franck–Condon (FC) factor. The UV absorption from these states sampled particular rovibronic levels of the potential well on the upper A 1Au state, disclosing transitions to FC active vibronic modes, involving couplings between torsion and cis-bend with CC stretch and trans-bend modes. These results suggest that the initial state preparation samples the bound rather than the purely repulsive region of the A 1Au state and that the rovibronic structure is the essential factor in affecting the absorption cross-section. They also indicate that the provided energy is insufficient to overcome the barrier on the A 1Au state, leading to the dominant non-adiabatic predissociative photofragmentation into C2H ( 2Σ+)+H.

Action spectroscopy and predissociation of vibrationally excited C2HD

Abstract H and D photofragments, produced via vibrationally mediated photodissociation of jet-cooled C2HD allowed measurement of vibrational and vibronic excitation action spectra of the parent as well as H and D Doppler profiles. The initial vibrational preparation at energies around 9700 cm−1 enhanced C—H and C—D bond cleavage for transitions from an initially excited bent state to the upper A~ state, rather than from a close lying state containing three C—H stretch quanta, due to favorable Franck—Condon factors for the former. The bound region of the A~ state was accessed, resulting in non-adiabatic dissociation and slight preferential C—H over C—D bond cleavage.