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Time-resolved Raman and absorption spectroscopies were used to characterize the vibrational frequencies and electronic structure of the *B1g excited state of nickel (II) octaethylporphyrin and nickel (II) protoporphyrin IX diemthyl ester in noncoordinating solvents. Spectroscopic evidence for structural relaxation processes within the initial *B1g excited state on picosecond time scales suggests structural interconversion. Multiple conformations are expected to exist in solution based on crystal structures and previous resonance Raman studies. Here we present calculations that support conformational interconversions. Structural dynamics may play a role in the complex mechanisms of many biological systems that involve metalloporphyrins.

Time-resolved absorption using pole wavelengths of 2.5-5.5 mu m has been used to study photoexcitations in the quasi-one-dimensional semiconductor, trans-polyacetylene. Present apparatus has 0.5-ps resolution, which is sufficient to observe the decay of the photogenerated charged soliton pairs which form after intrachain excitation of electron-hole pairs. The tunability of the apparatus permits the unambiguous identification of solitons and observation of spectral relaxation during their formation. >

Time resolved photoacoustic spectroscopy represents a promising new approach to the study of time resolved absorption with picosecond time resolution. The sensitivity of photoacoustic detection permits investigation of dilute and fluorescent samples which are difficult to study by conventional flash photolysis. Theory shows that quantitative measurements of important transient properties and new insights into photochemical and photophysical dynamics can be obtained by measuring heat deposition as a function of probe fluence. Experimental determinations of absorption and stimulated emission cross sections, and lower limits for rates of nonradiative relaxation of highly excited states, are presented.

The phase behavior of monolayer and multilayer Langmuir-Blodgett films of cadmium stearate on sapphire is studied using pulsed optoacoustic spectroscopy of the methyl and methylene stretching modes. Heating irreversibly disorders monolayer films but multilayers reach a stable configuration which undergoes reversible phase transitions.

The multiphoton ionization (MPI) spectra of trans‐1,3‐butadiene and three of its methyl substituted derivatives have been recorded using a tunable visible dye laser. The effect of methyl substitution on the electronic structure of butadiene is investigated, resulting in a reassignment of the spectra of these molecules. Quantum defects are found to be independent of principal quantum number, indicating little mixing between Rydberg and core orbitals. The validity of retaining conventional s, p, d, and f descriptions of Rydberg orbitals in these quasilinear molecules is demonstrated. Two electronic states of butadiene, assigned to the 3d and 4s Rydberg states, are observed for the first time in optical spectroscopy. No evidence for an excited 1Ag valence state is found in the MPI spectra.

Photoacoustic detection has been applied, for the first time, to detect and study excited molecules and reactive intermediates on the picosecond timescale. Dilute and weakly-absorbing transients are readily observed under conditions which render them inaccessible to conventional methods.

Direct measurements of the saturation absorption from Langmuir-Blodgett films of a polydiacetylene have been carried out in picosecond time domain. Using a photoacoustic technique, we have obtained directly the imaginary part of Kerr susceptibility, χ (3) (-ω;ω,ω, -ω). The magnitude of χ (3) is found to be in excess of 10 -8 esu and the associated ground state recovery time is less than the laser pulse width of 3 ps. These results are discussed in terms of a phase space filling model for a one dimensional exciton.

A kinetics model of multiphoton ionization (MPI) is developed to account for nonresonant ionization and collisional rethermalization effects on MPI. Simple quantitative criteria to describe MPI line shapes are derived. In cases where resonantly enhanced MPI (REMPI) is not an effective spectroscopic tool, it is demonstrated that absorption can deplete ground state levels which would otherwise have contributed to nonresonant ionization. The resulting structured valleys in the ionization current contain spectroscopic and dynamical information which may otherwise be unattainable. The feasibility of depletion of nonresonant ionization (DNRI) as a general technique is investigated and the first experimental observations of DNRI are reported and analyzed.

The photodissociation of M(CO) 6 , M = Cr or W, and the multiphoton ionization spectra of the resulting atoms M(I) have been studied using a tunable visible dye laser M(I) fragments are produced throughout the range between 363 and 585 nm, and multiphoton absorption allows all of the states formed to be probed.

Pulsed optoacoustic spectroscopy using a Raman-shifted tunable dye laser is used to record infrared absorption spectra of surface adsorbates between 2800 and 3800 cm −1 . The technique is sensitive to submonolayer coverages of OH and CH bond containing species on single crystal sapphire. The chemical interactions between adsorbed water (the initially dominant surface species), and trimethylaluminum (TMA) are studied. The effects of ultraviolet laser irradiation in vacuum and in ambient TMA vapor are investigated and used to draw conclusions about the mechanism of laser chemical vapor deposition of organometallics.

Piscosecond time-resolved absorption studies of the midgap absorption associated with photogenerated charged solitons in trans-polyacetylene are reported. Both direct photogeneration of charged-soliton pairs and neutral- to charged-soliton conversion are observed. The former are created by intrachain absorption and decay on a subpicosecond time scale. The latter are formed following interchain photoexcitation and are the dominant population of charged solitons at times greater than 20 ps.

The two‐photon fluorescence excitation (TPFE) spectra of regions of the ? states of two saturated amines 1‐azabicyclo [2.2.2]octane (ABCO) and trimethylamine (TMA) are reported. These spectra are compared with the respective one‐photon absorption (OPA), one‐photon fluorescence excitation (OPFE), and multiphoton ionization (MPI) spectra for both molecules. For ABCO, this comparison clearly indicates major differences in both the vibronic band intensities and the amount of sequence structure present in the TPFE spectrum relative to the MPI, OPA, and OPFE spectra, which are all comparable. The ’’distortions’’ of the TPFE spectrum are interpreted in terms of a laser‐induced ? state‐dependent depletion process from ? which results in ionization. Pressure effects on the TPFE spectrum imply that the rate of this up‐pumping process depends strongly upon the particular vibrational modes excited in the two‐photon‐induced ?←? transition. A further implication of this interpretation is that the intramolecular vibrati...

Time-resolved infrared spectroscopy with 0.5-ps resolution is used to track the evolution of the CO stretching vibration after visible photoexcitation of carboxyhemoglobin in water at room temperature. Polarization measurements determine that the iron-complexed CO is oriented nearly perpendicular to the porphyrin plane. The dissociation appears to proceed via a metastable excited state with 2 +/- 1 ps lifetime. The dissociated CO binds weakly in the heme pocket for at least 500 ps. This state correlates with the internally bound state observed by Frauenfelder et al. at low temperatures in myoglobin.

Multiphoton dissociation of Mn2(CO)10 followed by multiphoton ionization of the generated fragments is accomplished with a single tunable pulsed dye laser. Efficient dissociation to bare Mn atoms occurs over a broad range of wavelengths via a sequential photodissociation mechanism. Spectral changes upon addition of buffer gases are shown to reflect collisional effects on intermediates in the dissociation process.

In this paper we report the development and application of photoacoustic spectroscopy (PAS) for the detection and characterization of transient intermediates with lifetimes on the picosecond timescale [1].

The multiphoton ionization (MPI) spectra of trans‐1,3‐butadiene and three of its methyl substituted derivatives have been recorded using a tunable visible dye laser. The effect of methyl substitution on the electronic structure of butadiene is investigated, resulting in a reassignment of the spectra of these molecules. Quantum defects are found to be independent of principal quantum number, indicating little mixing between Rydberg and core orbitals. The validity of retaining conventional s, p, d, and f descriptions of Rydberg orbitals in these quasilinear molecules is demonstrated. Two electronic states of butadiene, assigned to the 3d and 4s Rydberg states, are observed for the first time in optical spectroscopy. No evidence for an excited 1Ag valence state is found in the MPI spectra.

The two‐photon fluorescence excitation (TPFE) spectra of regions of the ? states of two saturated amines 1‐azabicyclo [2.2.2]octane (ABCO) and trimethylamine (TMA) are reported. These spectra are compared with the respective one‐photon absorption (OPA), one‐photon fluorescence excitation (OPFE), and multiphoton ionization (MPI) spectra for both molecules. For ABCO, this comparison clearly indicates major differences in both the vibronic band intensities and the amount of sequence structure present in the TPFE spectrum relative to the MPI, OPA, and OPFE spectra, which are all comparable. The ’’distortions’’ of the TPFE spectrum are interpreted in terms of a laser‐induced ? state‐dependent depletion process from ? which results in ionization. Pressure effects on the TPFE spectrum imply that the rate of this up‐pumping process depends strongly upon the particular vibrational modes excited in the two‐photon‐induced ?←? transition. A further implication of this interpretation is that the intramolecular vibrati...

We discuss two important aspects of the nonlinear optical response of polyacetylene, the prototype conjugated polymer. In the subband gap region, χ(3) of polyacetylene has a rich spectral response and its magnitude is found to be the largest among semiconductors. In the superband gap region, absorption of photons leads to direct photogeneration of charged solitons on a femtosecond-time-scale. These results are discussed in the context of a one-dimensional, coupled electronlattice system, and are generalized to other conjugated polymers.

Hanle resonances for coherent four-wave mixing in the ground state of Na are reported whose intensity increases and whose linewidth decreases with increasing inert buffer gas pressure. In 10-mTorr Na vapor with 4 atm of argon, resonances of less than 35 mG full width at half maximum are observed. The high resolution permits direct observation of the fundamental homogeneous broadening mechanisms for the Zeeman coherences responsible for four-wave mixing.

Four-wave light scattering experiments have been carried out, in mixtures of sodium vapor (35 mTorr partial pressure) in buffer gases of He and/or N2 (partial pressure up to 103 Torr).

Measurements of the dynamics of the photoinduced 0.45-eV midgap absorption associated with photogenerated charged solitons in trans-polyacetylene are reported. We have studied the intensity, polarization, and temperature dependences of the dynamics of this band with 5-ps resolution. Two types of charged solitons are clearly identified. First, there are directly photogenerated Su-Schrieffer soliton pairs which form following intrachain absorption and geminately recombine on subpicosecond time scales. At later times (\ensuremath{\gtrsim}20 ps), there are charged solitons formed by an interchain absorption mechanism where polarons are formed and diffuse to convert residual neutral solitons to charged solitons.

We study phenylenevinylene chromophore pairs whose spacing and orientation can be varied by coupling them to peptide backbones. Copious interchromophore excitations are observed and explain reduced fluorescence yields in conjugated polymer films relative to solutions.

We have studied the temporal profile of photon-echo signals generated by combined gated cw and pulsed dye-laser excitation of the inhomogeneously broadened, 555.6-nm absorption line of (174)Yb vapor. We find that the echo profile is, after time reversal, essentially identical with that of the first excitation pulse. We give a new analysis of this effect. Since time-reversed pulse reproduction should also occur in inhomogeneously broadened solid samples, and since we observe time-reversed reproduced pulses up to 4% as intense as the input pulse, this effect may have important applications in optical signal-processing systems.

A technique to record transient vibrational spectra with 0.5-psec resolution is reported. The apparatus is based on seeded optical parametric amplification. A study of the microscopic details of the photodissociation of carboxyhemoglobin is described as an example.

We describe the generation of 350-fs pulses tunable between 2.3 and 5.5 microm by seeded optical parametric amplification. These pulses are synchronized with visible pulses for time-resolved absorption and reflectivity in semiconductors with 500-fs resolution.