Spectroscopy of infrared multiphoton excited molecules

Multiplex CARS study of infrared-multiphoton-excited OCS, at Ninth International Conference on Laser Spectroscopy (Bretton Woods, NH), Thursday, June 1, 1989
The vibrational energy distribution following the ?2 overtone excitation of OCS by a pulsed CO2 laser can be studied by coherent anti-Stokes Raman spectroscopy (CARS) of the ?1 mode. Since the anharmonicity of the pumped mode is small, and since the relaxation to other modes is slow, OCS allows one to study the interaction of an isolated harmonic mode with an intense laser field. Single-shot spectra are obtained by using a broadband dye laser and recording the CARS signal on a multichannel analyzer. The phase matching of the beams is done in a so-called folded boxcars arrangement. The... Read more about Multiplex CARS study of infrared-multiphoton-excited OCS
E. N. Glezer. 1996. “Ultrafast electronic and structural dynamics in solids”. Publisher's VersionAbstract
This thesis investigates the dynamics of electrons and atoms in solids driven by intense, ultrashort laser pulses. Results of two series of experiments are presented. In the first set, the changes in the electronic properties of the semiconductor GaAs are determined by measuring the changes in its optical properties in response to 70-fs laser pulses. A fluence range of up to, and above, the damage threshold is examined. The experiments differ from previous work in the field, in that they are direct time-resolved measurements of the dielectric function and second- order optical susceptibility fundamental quantities that characterize the optical state of the material. The dielectric function is measured from 1.5 to 3.5 eV, and at 4.4 eV, while the second-order susceptibility is measured at a single frequency of 2.2 eV. The results suggest a new view of the underlying electronic and structural changes. Three regimes of behavior are observed: at low excitation, rapid bandstructure changes are followed by lattice heating for about 10 ps; at medium excitation, stronger bandstructure changes are followed by a loss of long-range order in the crystal within several picoseconds; and at high excitation, an increasingly rapid transition to a metallic state is seen. In the second set of experiments, the effect of ultrafast excitation inside the bulk of a solid is studied. It is shown that submicron-diameter voxels can be produced inside many transparent materials by tightly focusing 100-fs laser pulses. The use of such voxels for high-density 3D optical data storage is demonstrated. Scanning electron microscopy and atomic force microscopy are used to examine 200-nm diameter voxels. The results suggest that extreme temperatures and pressures create a micro-explosion, leading to the formation of a void surrounded by densified material. Permanent structural changes are produced even in such hard materials as quartz and sapphire.
J. Paul Callan, A. M.-T. Kim, L. Huang, E. N. Glezer, and E. Mazur. 1997. “From semiconductor to metal in a flash: observing ultrafast laser-induced phase transformations.” In . Materials Research Society Fall Meeting. Publisher's VersionAbstract
We use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of a material over the spectral range 1.53.5 eV following intense 70-fs laser excitation. The results reveal the nature of the phase transformations which occur in the material following excitation. We studied the response of GaAs and Si. For GaAs, there are three distinct regimes of behavior as the pump fluence is increased lattice heating, lattice disordering, and a semiconductor-to-metal transition.
C. B. Schaffer, A. Brodeur, N. Nishimura, and E. Mazur. 1999. “Laser-induced microexplosions in transparent materials: microstructuring with nanojoules.” In . Photonics West. Publisher's VersionAbstract
We tightly focus femtosecond laser pulses in the bulk of a transparent material. The high intensity at the focus causes nonlinear absorption of the laser energy, producing a microscopic plasma and damaging the material. The tight external focusing allows high intensity to be achieved with low energy, minimizing the effects of self-focusing. We report the thresholds for breakdown and critical self-focusing in fused silica using 110-fs pulses at both 400-nm and 800-nm wavelength. We find that permanent damage can be produced with only 10 nJ (25 nJ) for 400-nm (800-nm) pulses, and that the threshold for critical self-focusing is 140 nJ for the 400-nm pulses and 580 nJ for the 800-nm pulses. The critical self-focusing thresholds are more than an order of magnitude above the breakdown thresholds, confirming that self-focusing does not play a dominant role in the damage formation. This lack of self-focusing allows a straightforward interpretation of the wavelength and bandgap dependence of bulk breakdown thresholds. The energies necessary for material damage are well within the range of a cavity-dumped oscillator, allowing for precision microstructuring of dielectrics with a high repetition-rate laser that is roughly one-third the cost of an amplified system.
K. Hsien Chen, C. Lu, L. Anibal Avils, E. Mazur, N. Bloembergen, and M. J. Shultz. 1989. “Multiplex coherent anti-Stokes Raman spectroscopy study of infrared-multiphoton-excited OCS.” J. Chem. Phys., 91, Pp. 1462–1468. Publisher's VersionAbstract
The vibrational energy distribution following v2 overtone excitation of OCS by a pulsed CO2 laser is studied by monitoring the coherent anti-Stokes Raman spectrum of the v1 mode. Because of the slow energy transfer from the pumped mode to other modes, and because the anharmonicity of the v2 mode is small, OCS is an ideal system for studying the interaction of an intense infrared laser field with a single, nearly harmonic, oscillator. From the spectra the cross-anharmonicities of the of the n1 mode are determined to be x12 = -6.0 cm-1 and x13 = -2.7 cm-1, respectively. The time-dependence of the spectra provides information on V-V energy transfer rates. In particular, the measurements put a lower limit of kv2->v2 = 1 ms-1 torr-1 on the vibrational relaxation rate within n2 mode. At high excitation, the temperature of the v2 mode rises up to 2000 K, and hot bands are observed up to the v = 4 level. This fourth overtone peak is split because of either a Fermi resonance or vibrational angular momentum splitting.
J. Wang, K. Hsien Chen, and E. Mazur. 1987. “Highly nonthermal intramolecular energy distribution in isolated infrared multiphoton excited CF2Cl2 molecules.” In Laser Spectroscopy VIII, edited by S. Svanberg, Pp. 236–238. Springer-Verlag. Publisher's VersionAbstract
When a polyatomic molecule with a strong vibrational absorption band is irradiated with an intense resonant infrared laser pulse it can absorb many (10 to 40) infrared photons. If some initial energy deposition is localizedpreferably in one vibrational mode or in a subset of modesit may become possible to induce mode-selective reactions by infrared multiphoton excitation. The intramolecular dynamics of infrared multiphoton excited molecules has been studied by a variety of spectroscopic techniques. One of these techniques is spontaneous Raman spectroscopy. In the past five years this technique has been successfully applied to monitor the vibrational energy in infrared multiphoton excited molecules. In this work we present experimental results of recent time-resolved spontaneous Raman experiments on collisionless infrared multiphoton excited CF2Cl2 molecules. The experiments show that the intramolecular energy distribution is highly nonthermal, and that a large part of the vibrational energy remains localized in the pump mode for a period of time long compared to the mean free time of the molecules.
C. Lu, J. R. Goldman, S. Deliwala, K. Hsien Chen, and E. Mazur. 1991. “Direct evidence for nu1-mode excitation in the infrared multiphoton excitation of SO2.” Chem. Phys. Lett., 176, Pp. 355–360. Publisher's VersionAbstract
We investigated the infrared multiphoton excitation of SO2 in bulk samplesand in a supersonic jet with the 9R(22), 9R(32), and 9P(32) CO2-laser lines. Coherent anti-Stokes Raman spectra reveal unambiguously that only the nu1-mode at 1151.3 cm-1 is actually pumped; no 2nu2 overtone pumping at 1035.2 cm-1 is observed. From the spectra we directly determine the anharmonic constants chi11 = -3.65 ± 0.06 cm-1 and chi12 = -3.3 ± 0.3 cm-1.

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