George N. Gibson

Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gases

Measurements of the vacuum-ultraviolet (<80-nm) radiation produced by intense ultraviolet (248-nm) irradiation (1015–1016 W/cm2) of rare gases have revealed the copious presence of both harmonic radiation and fluorescence from excited levels. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen from ions of Ar, Kr, and Xe, with the shortest wavelengths observed being below 12 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d95s5p → 4d105s manifold of Xe7+ at ~17.7 nm, was detected. These experimental findings, in alliance with other studies concerning multielectron processes, give evidence for a role of electron correlations in a direct nonlinear process of inner-shell excitation.

Electro-optically cavity-dumped ultrashort-pulse Ti:sapphire oscillator

Summary form only given. Acousto-optical cavity-dumping (AO) has been demonstrated with short-pulse Ti:sapphire lasers producing single-pulse energies of 62 nJ. However, AO cavity-dumping is inherently limited because the AO wave must traverse the beam waist in the round-trip time of the cavity limiting the beam waist to roughly 30 /spl mu/m. This limits the pulse energy as the intensity must be kept below the threshold for nonlinear processes in the AO crystal. Furthermore, the contrast in the pulse energy on either side of the main cavity-dumped pulse is rather low (30:1 or 20:1). Electro-optical (EO) cavity dumping has no such limitation and can be scaled to higher energies with high contrast. However, there are several difficulties associated with EO cavity dumping all of which have now been solved.

Ultrafast time-resolved carotenoid to-bacteriochlorophyll energy transfer in LH2 complexes from photosynthetic bacteria.

Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment-protein complexes isolated from three different strains of photosynthetic bacteria: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of pi-electron conjugated carbon-carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at approximately 90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to approximately 53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S1 (2(1)Ag-)-->S2 (1(1)Bu+) excited-state transitions which, when subtracted from the energies of the S0 (1(1)Ag-)-->S2 (1(1)Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (2(1)Ag-) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car*+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S1 (1(1)Ag-), S2 (1(1)Bu+), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S1 (2(1)Ag-) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of pi-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon-carbon double bonds in LH2 complexes from purple photosynthetic bacteria.

Ultrafast time-resolved spectroscopy of xanthophylls at low temperature.

Many of the spectroscopic features and photophysical properties of xanthophylls and their role in energy transfer to chlorophyll can be accounted for on the basis of a three-state model. The characteristically strong visible absorption of xanthophylls is associated with a transition from the ground state S0 (1(1)Ag-) to the S2 (1(1)Bu+) excited state. The lowest lying singlet state denoted S1 (2(1)Ag-), is a state into which absorption from the ground state is symmetry forbidden. Ultrafast optical spectroscopic studies and quantum computations have suggested the presence of additional excited singlet states in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+). One of these is denoted S* and has been suggested in previous work to be associated with a twisted molecular conformation of the molecule in the S1 (2(1)Ag-) state. In this work, we present the results of a spectroscopic investigation of three major xanthophylls from higher plants: violaxanthin, lutein, and zeaxanthin. These molecules have systematically increasing extents of pi-electron conjugation from nine to eleven conjugated carbon-carbon double bonds. All-trans isomers of the molecules were purified by high-performance liquid chromatography (HPLC) and studied by steady-state and ultrafast time-resolved optical spectroscopy at 77 K. Analysis of the data using global fitting techniques has revealed the inherent spectral properties and ultrafast dynamics of the excited singlet states of each of the molecules. Five different global fitting models were tested, and it was found that the data are best explained using a kinetic model whereby photoexcitation results in the promotion of the molecule into the S2 (1(1)Bu+) state that subsequently undergoes decay to a vibrationally hot S1 (1(1)Ag-) state and with the exception of violaxanthin also to the S* state. The vibrationally hot S1 (1(1)Ag-) state then cools to a vibrationally relaxed S1 (2(1)Ag-) state in less than a picosecond. It was also found that a portion of the S* population is converted into S1 (2(1)Ag-) during deactivation, but this process and the relative yield of S* was found to depend on temperature, consistent with it being associated with a twisted conformation of the xanthophyll. The results of the global fitting suggest that subpopulations of twisted conformers of xanthophylls already exist in the ground state prior to photoexcitation.

Ultrahigh-intensity KrF* laser system

The operational characteristics of an ultrahigh-intensity subpicosecond large-aperture KrF* laser system are described. Measurements show the achievement of a focal spot diameter of less than 1.7 microm. Combined with measurements of the pulse width and pulse energy, this yields an average intensity of ~2 x 10(19) W/cm(2), a value corresponding to a peak electric field of ~24 (e/a(0)(2)). Light sources of this nature will find application in a broad range of studies of the nonlinear properties of matter in the strong-field regime.

Measurement of 248-nm subpicosecond pulse durations by two-photon fluorescence of xenon excimers

A technique for measuring the duration of single ultrashort pulses at KrF(*) wavelengths has been developed that employs fluorescence from xenon excimers excited by two-photon absorption of atom pairs. Pulses of ~350-fsec duration have been measured at 248 nm.

Dispersion-free transient-grating frequency-resolved optical gating

We have developed a compact dispersion-free TG (transient-grating) FROG (frequency-resolved optical gating) by utilizing a mask that separates the input beam into three distinct beams focused into fused silica to create the FROG signal. Two of the beams are reflected off the same set of mirrors to ensure identical optical paths, eliminating the difficulty in establishing zero time delay between the beams. In addition, the use of only reflective optics avoids material dispersion in the FROG except for the mixing crystal. This TG FROG is capable of operating with an intensity of 1 x 10(11) W/cm(2) and has resolutions less than 0.5 and 1.3 fs for 25- and 10-fs input pulses, respectively.

Measurement of energy penetration depth of subpicosecond laser energy into solid density matter

The energy penetration depth characteristic of the interaction of intense subpicosecond (∼600 fs) ultraviolet (248 nm) laser radiation with solid density material has been experimentally determined. This was accomplished by using a series of ultraviolet transmitting targets consisting of a fused silica (SiO2) substrate coated with an 80–600 nm layer of MgF2. The measurement of He‐like and H‐like Si and Mg lines, as a function of MgF2 thickness, enabled the determination of the energy penetration depth. It was found that this depth falls in the range of 250–300 nm for a laser intensity of ∼3×1016 W/cm2. Based on numerical simulations, it is estimated that solid density material to a depth of ∼250 nm is heated to an electron temperature of ∼500 eV.

Fifth-harmonic production in neon and argon with picosecond 248-nm radiation

The results of a study of fifth-harmonic production in neon and argon irradiated with 248-nm picosecond laser pulses are presented. Focused intensities range from 1013 to 1015 W/cm2. Data for fifth-harmonic intensity as a function of both target density and focused laser intensity are presented and compared with theory. For the laser intensities and medium densities studied, estimates for the linear and nonlinear components of Δk, the wave-vector mismatch between the fundamental and harmonic waves, indicate that the nonlinear component is much greater than the linear component.

Tunneling Ionization Rates from Arbitrary Potential Wells

We present a practical numerical technique for calculating tunneling ionization rates from arbitrary 1D potential wells in the presence of a linear external potential by determining the widths of the resonances in the spectral density, {rho}(E) , adiabatically connected to the field-free bound states. While this technique applies to more general external potentials, we focus on the ionization of electrons from atoms and molecules by dc electric fields, as this has an important and immediate impact on the understanding of the multiphoton ionization of molecules in strong laser fields. {copyright} {ital 1998} {ital The American Physical Society}