Nonlinear intensity dependence in the infrared multiphoton excitation and dissociation of methanol pre-excited to different energies

Abstract

We report quantitative dissociation yields for the reaction CH 3 OH (v OH) → nh CH 3 ϩOH induced by infrared multiphoton excitation of methanol pre-excited to various levels of the OH stretching vibration (v OH ϭ0, 1, 3, 5). The yields are measured by detecting OH using laser induced fluorescence. It is demonstrated that for low levels of pre-excitation (v OH ϭ0, 1, 3) there is a substantial nonlinear intensity dependence, as a higher yield is found for self mode-locked CO 2 laser pulses ͑with higher peak intensity͒ as compared to single mode pulses of the same laser fluence, but lower peak intensity. In contrast, at high levels of preexcitation (v OH ϭ5) this nonlinear intensity dependence is absent. Quantitative model calculations are carried out using a case B/case C master equation approach that takes nonlinear intensity dependence into account. The calculations are consistent with the experimental results and confirm the prediction that an important part of the selectivity of the CO 2 laser excitation step in infrared laser assisted photofragment spectroscopy of CH 3 OH is due to this nonlinear intensity dependence. We discuss further consequences of these experimental observations and theoretical predictions, which are also extended to infrared multiphoton excitation of C 2 H 5 OH. Infrared ͑C-O͒ chromophore band strengths are reported for CH 3 OH and C 2 H 5 OH.

Controlled modification of biomolecules by ultrashort laser pulses in polar liquids

“…This may result from the scattering of laser light by micro-bubbles within the laser beam formed due to laser-stimulated decomposition of the solvents. The observed modification of methanol, accompanied by micro-bubble generation, and the stability of water are both consistent with previously published results on femtosecond-pulse laser modification of those liquids 20 – 22 . In particular, the laser intensity required to produce micro-bubbles in water was about 10 13 W/cm 2 20 , 21 , which was not attainable in our experiments.…”

“…Following the commonly accepted concept 20 – 22 we attribute the generation of micro-bubbles to the process of solvent ionization followed by possible chemical modification by the laser pulses. It had a threshold-type nonlinear dependence on laser intensity/fluence: for each solvent, the modification took place only at laser intensities exceeding a certain level (referred to as threshold of solvent modification) characteristic for each liquid.…”

“…Other possible modification paths can include breaking of the CO bonding due to partial energy transfer from the OH stretch to harmonics of the CO stretch 28 – 30 . It is remarkable that excitation of the OH stretch followed by energy transfer to the CO stretch is favorable for dissociation of methanol molecules into CH 3 and OH fragments 17 , 22 . The active OH fragment of the dissociated methanol molecule could be attached to a positively charged peptide side-chain to produce the reported Met oxidation of the peptide in methanol.…”

Parity‐Violating Potentials for the Torsional Motion of Methanol (CH3OH) and Its Isotopomers (CD3OH, 13CH3OH, CH3OD, CH3OT, CHD2OH, and CHDTOH)

“…4 and 5. In the case of CHD 2 OH, we refer to three transition-state structures (1, 3, 5) and three equilibrium structures (2,4,6), as indicated in Fig. 4.…”

“…Introduction. ± Among the many facets of research on MeOH, which range from its large-scale industrial production and use as a propellant, requiring its statistical thermodynamics and kinetics of combustion [1], to its prototype role in infrared multiphoton excitation and dissociation [2] and, finally, its high-resolution spectroscopy, intramolecular vibrational redistribution [3], and tunneling dynamics [4], certainly a particularly intriguing result was the synthesis and characterization of the chiral isotopomer CHDTOH by Arigoni and co-workers [5] [6]. This molecule can also be considered to be one of the simplest representatives of XOCR 1 R 2 R 3 type of molecules, which have been proposed for the possible construction of an absolute molecular clock (using also the antimatter enantiomers [7]).…”