Spectroscopy of infrared multiphoton excited molecules

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.

We use self-phase modulation in a single-mode fiber to produce broadband femtosecond laser pulses. Subsequent amplification through two Bethune cells yields high-energy, tunable, pulses synchronized with the output of an amplified colliding-pulse-modelocked (CPM) laser. We routinely obtain tunable 200-J pulses of 42-fs (FWHM) duration with a nearly perfect Gaussian spatial profile. Although self-phase modulation in a single-mode fiber is widely used in femtosecond laser systems, amplification of a figer-generated supercontinuum in a Bethune cell amplifier is a new feature which maintains the high-quality spatial profile while providing high gain. This laser system is particularly well suited for high energy dual-wavelength pump-probe experiments and time-resolved four-wave mixing spectroscopy.

Light beating spectroscopy has been used from the early days of the laser to study light scattering. By detecting the beating signal between the scattered light and a 'local oscillator' field derived from the same laser, resolving powers of 10^14 have been achieved. The Fourier transform heterodyne spectroscopy presented here is simpler and more direct than the conventional heterodyne techniques using autocorrelators or spectrum analyzers.

Submicron-diameter structures can be produced inside many transparent materials by tightly focused 100-fs laser pulses. The ultrafast energy deposition creates very high temperature and pressure inside the region, initiating a microexplosion. Material is ejected from the center and forced into the surrounding volume, forming a void surrounded by densified material. Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.

We used water and human skin tissue to compare the surgical potential of 100-fs and 200-ps laser pulses. For investigation of threshold behavior of 100-fs and 200-ps pulses, we use water as a model for tissue. In addition to having a lower threshold, we find that energy deposition is much more consistent with 100-fs pulses. We also compared 100-fs and 200-ps laser pulse effects on the surface and in the bulk of human skin tissue. On the surface, pulses with 100-fs and 200-ps duration leave similar size ablation regions. In the bulk both 100-fs and 200-ps pulses produce cavities, however, 100-fs pulses result in a smaller cavity size. On both the surface and in the bulk 100-fs pulses show less collateral tissue damage than 200-ps pulses.

This thesis presents the results of three experiments which use lasers to investigate energy-transfer and charge-transfer dynamics. The dynamical processes studied include nanosecond vibrational energy transfer in molecules, subpicosecond electron relaxation in semiconductors, and subpicosecond initiation of surface bimolecular reactions on a metal crystal. In experiments using time-resolved coherent Raman spectroscopy to probe infrared multiphoton excited molecules, we study CO 2-laser excited SO2 and SF6. In SO2 we observe direct n1-mode excitation and distinguish between this process and excitation of the nearly resonant n2-mode overtone. In SF6, we directly observe n3-mode excitation followed by collisional energy redistribution to a heat bath of non-pumped modes. Quantitative modeling of the SF6 spectra yields excited vibrational population distributions and resolves some long-standing inconsistencies between different previously published reports. In an experiment using time-resolved photoelectron spectroscopy, we observe the subpicosecond evolution of an optically-excited nonequilibrium electron distribution in silicon. We observe an electron thermalization time of less than 120 fs, electron equilibration with the lattice in 1 ps, and an energy-dependent electron cooling rate consistent with published calculations of the electron-phonon scattering rate. The results indicate the formation, in 1 ps, of a surface space-charge electron layer with an electron density two orders of magnitude greater than the bulk electron density. In an experiment using 100-fs laser pulses to induce desorption of O2 and reaction of O2+CO to form CO2 on a Pt(111) surface, we present desorption and reaction data obtained over an absorbed fluence range of 1- 20 mJ/ cm2 at wavelengths of 800, 400, and 266 nm. We observe a highly nonlinear desorption and reaction yield fluence dependence; the data are fit by a power law model in which the yield is proportional to fluence to the power p = 5.9 and 3.8 for the 800 nm and 400 nm data respectively. The ratio of O2 to CO2 desorption is found to be 14:1, 12:1 and 3:1 at 800, 400, and 266 nm respectively. At 800 nm, the desorption and reaction are independent of laser pulsewidth in the range 100 fs to 3.6 ps. Finally, this thesis describes the design, development and operation of new equipment used for the surface reaction experiment: a state-of-the- art amplified femtosecond Ti:sapphire laser, and an ultrahigh-vacuum surface- science chamber.

A reply to a Comment by A.L. Malinovsky and E.A. Ryabov on an article in the Physical Review Letters by Chen, Wang and Mazur. Their comment is attached to this reply.

Multiphoton excitation and dissociation of DN3 by short CO2 laser pulses is shown to be a collisionless process. The characteristic features of this multiphoton process are systematically studied. The average number of photons absorbed per DN3 molecule and the absolute dissociation yield show a strong dependence on the peak laser intensity. Resonantly enchanced coherent multiphoton excitation, rather than stepwise incoherent excitation, is suggested. The primary dissociation products of DN3 are ND[1delta] and N2. Formation of vibrationally excited ND[1delta] intermediates is suggested. The reactions of ND[1delta] with DN3 lead to chemiluminescent signals originating from the formation of electronically excited ND2[2A1] and ND[3II]. Formation of the ND[3II] intermediate is attributed to a reaction of ND[1delta] and vibrationally excited DN3 molecules: ND[1delta]+DN3-ND[3II]+ND[3-]+N2.

Abstract not available online.

Pure rotational coherent anti-Stokes Raman spectroscopy using a single broadband dye-laser has been applied to study rotational energy distribution of molecules in a pulsed supersonic beam. The multiplex BOXCARS configuration allows accurate determination of rotational constants and rotational temperatures, and offers a great number of advantages over other methods. The technique was applied to calibrate the cooling effect of a pulsed molecular beam of N2 and to study the rotational energy distributions of infrared multiphoton excited C2H4.

We studied the effect of self-phase modulation and self-focusing on transient stimulated Raman scattering in high-pressure hydrogen by using high-energy, subpicosecond laser pulses. Adding argon to the hydrogen emphasizes the effect of self-phase modulation on stimulated Raman scattering by increasing the former effect without affecting the latter. The behavior of the observed stimulated Raman scattering falls into three regimes depending on input energy: normal stimulated Raman scattering at low energies, suppression by self-phase modulation at medium energies, and a recovery at high energies because strong self-focusing limits self-phase modulation.

This paper presents an overview of recently obtained results on infrared multiphoton excited molecules using coherent anti-Stokes Raman Spectroscopy. The data underline the important role of collisions in the excitation process.

Using tightly focused 100-fs, 800-nm laser pulses we produce breakdown in water using less than 1 J of energy. By imaging the cavitation and pressure wave propagation we find that the supersonic expansion is limited to an 11-m diameter for 1-J pulses, increasing to a 20-m diameter for 30-J pulses.