Following a brief description of the double beam picosecond spectrometer, we present two examples taken from photochemistry and photobiology. At first, results are presented on the nonradiative electronic relaxation (internal conversion and intersystem crossing) of photoexcited acridine. Subsequently, the nature of the primary steps in photosynthesis is considered. Picosecond absorption spectroscopy results are given that suggest a new mechanism for the photooxidation of bacteriochlorophyll.
A picosecond emission apparatus utilizing a mode-locked Na+3/phosphate glass laser and a streak-camera-optical-multichannel-analyzer detection system is described, and some general classes of experiments in this research area are discussed. Special emphasis is placed on nonexponential decays. A new type of diffusion controlled chemical reaction based on rotational diffusion is proposed for study using picosecond fluorescence depolarization methods. These methods are also useful for the assessment of solvent dependent molecular shape changes of flexible molecules and the relationship these structural changes might have with solvent dependent chemical reactivity. The study of fluorescence probe molecules on surfaces or in biological membranes or macromolecules is described. The picosecond fluorescence depolarization technique extends the study of rotation of whole macromolecules or polymers to small molecules or to small molecular segments in a macromolecule or a polymer. Picosecond emission spectroscopy, combined with picosecond absorption spectroscopy of the solvated electron, allows assessment of one-photon photoionization as a primary process in photochemistry and photobiology. A tentative result indicates that tryptophan photionizes both by direct electron ejection and by an intramolecular scavenging effect. Because of slow interchange between conformations that can and cannot lead to intramolecular scavenging, a double exponential is observed. Picosecond emission and depolarization studies of the fluorescence probes ANS and TNS give direct evidence for strong solvent attachment effects in the excited electronic state. A sort of "crystallization" of water molecules around the solute occurs in the time range of about 50 psec and is D20 dependent. ANS and TNS in mixed water/ethanol solvents show nonexponential decays which are interpreted in terms of site inhomogeneities in the liquid state. This discovery suggests that studies of wavelength dependent emission decays and picosecond "hole burning" experiments are feasible in the liquid state.
This paper reports recent results in the development of a picosecond short cavity dye laser designed to operate in the 400-500 nm range. The short cavity dye laser is axially pumped with a single Nd+3: glass third harmonic pulse of 11 ps. The output pulse duration, measured with a light gate, is less than 13.8 ps. A theoretical analysis of the laser rate equations is presented which indicates that this blue output pulse may be as short as 1.5 ps and transform limited. single and multiple mode operation are reported with individual mode bandwidths of ≈0.2 nm. Gross tuning of the laser is accomplished by changing dyes and demonstrated for the spectral region of 396 nm to 470 nm. Finer tuning is accomplished by changing the cavity length, while the finest wavelength selection is produced by changing the pumping angle. The maximum energy conversion efficiency measured is ≈6%. This laser should be useful as a broadband source for picosecond spectroscopy of solutions or a nar-rowband source for excitation of gases or low temperature crystals. No practical source of high-power, tunable picosecond pulses in the spectral region of this laser has been previously reported.
Picosecond photodynamics of triphenylmethyl cation dyes are examined in the visible spectral region with time and wavelength resolved spectroscopy. Computer modeling of the data is employed to determine recovery times for ground state absorption. Previous work is reviewed and compared to the current results. It is proposed that dielectric relaxation is the controlling step in the return of ground state absorption, rather than viscosity as proposed by previous authors. Data is presented from studies in the field of dielectric relaxation to support this contention.
This paper presents our recent work on coherent optical spectroscopy of molecules and molecular beams. The theory for these nonlinear optical effects is summarized and related to the measurements in the gas phase and in the condensed phase. Finally, we discuss the importance of these methods, which disentangle the inhomogeneous optical resonances, in understanding nonradiative and optical dephasing processes.
We briefly review the areas of luminescence line-broadening and energy transfer in amorphous solids, with particular emphasis on non-resonant phonon-assisted energy transfer between like ions. The laser-induced fluorescence line-narrowing technique is discussed, and the importance of resonant excitation-detection is emphasized. We illustrate the discussion with several examples from our recent work. A two-phonon Raman scattering propess is shown to be the dominant source of homogeneous thermal line-broadening of the 5D0→ 7F0 fluorescence line of Eu 3+ in a silicate glass, over the temperature range 200-350 K. We have further shown that the scattering efficiency is site-dependent, correlating with the static crystal field strength. Our results on spectral diffusion (energy transfer between like ions having different transition energies) indicate an electric dipole-dipole coupling mechanism between Eu3+ ions in a phosphate glass. The phonon involvement was determined to be an intersite two-phonon Raman process, with one-phonon action at each of the two coupled ion sites.
Saturated Optical Nonresonance Emission Spectroscopy (SONRES) has been applied to metallic vapor detection. Using one-photon cw excitation less than 0. 2 atoms of sodium have been detected in the laser interaction volume with an ambient pressure environment. The method has been applied to the ultraviolet transitions of nickel and platinum using pulsed excitation sources, achieving detection limits less than 100 atoms in the detection volume for an ambient pressure high temperature environment. Resonance enhanced two-photon excitation has also been investigated for atomic detection applications. Experiments demonstrate that the two-photon method can be expected to achieve ultimate detection limits of 10-3 atoms in the laser interaction volume, approaching the theoretical limit of the one-photon SONRES method. The two-photon method provides optimum performance at low pressure and furnishes the sub-Doppler resolution required for highly selective isotope analysis.
Optical heterodyne methods significantly modify the signal-to-noise (S/N) considerations for Coherent Raman Effects (CRE). Various CRE phenomena are examined and S/N considera-tions are introduced. Specific methods considered include Stimulated Raman Gain (SRG), Inverse Raman Scattering (IRS), Coherent Anti-Stokes Raman Spectroscopy (CARS), Coherent Stokes Raman Spectroscopy (CSRS), background suppression techniques with four wave mixing (SUBMARINE and ASTERISK), and Raman Induced Kerr Effect Spectroscopy (RIKES). The use of heterodyne mixing is illustrated with RIKES, experimental results are presented, and a com-parison of the S/N expected for the various techniques is introduced.
Polarization spectroscopy makes use of the polarization dependence of the nonlinear interaction between two laser beams in a gaseous medium. The laser-induced optical anisotropy is calculated using a rate equation approach, and the effect of this anisotropy on a polarized probe beam is derived. The method is useful for Doppler-free spectroscopy, for similification of molecular spectra, and for relaxation studies. A comparison with other Doppler-free saturation spectroscopy methods shows an advantage in signal-to-noise for polarization spectroscopy. Recent high resolution experiments with hydrogen, molecular sodium, and nitrogen dioxide are presented.
This paper reviews the current "state of the art" of the theoretical understanding of multiphoton photo-fragmentation of collision-free molecules. Some problems pertaining to the molecular level structure, to intramolecular dynamics and to the effects of the interaction of intense electromagnetic fields with molecular systems are considered.
Multiple infrared photon absorption is a quite general process which molecules can undergo when placed in a high flux of infrared energy, such as the focussed beam of a CO2 laser. Among the important consequences of this kind of absorption are isotopically selective molecular dissociation and stimulation of specific chemical reactions. In order to understand how this process works, we must be able to follow the evolution of the molecules through their internal states, populated by photon absorption. Double resonance spectroscopy is the method of a choice for getting at this information. A system pumped by CO2 laser radiation can be examined with a tunable laser probe beam, such as that from a lead-salt diode laser. From such an experiment, we can directly observe Rabi modulation of the absorption lines, determine elementary state-to-state relaxation pathsways, and locate higher excited vibrational states. Systems currently under investigation include SF6 and vinyl chloride. In suitable cases, the probe beam can be a tunable visible or u.v. source, such as a dye laser. Fluorescence spectroscopy can then be used to monitor the transient absorptions produced by multiple-photon excitation. Among the systems which can be examined in this way are biacetyl and glyoxal.
An isotopically specific transverse discharge chemical laser is used to excite single isotopes of HBr (79-81), DBr (79-81), HCℓ (35-37), or DCℓ (35-37). Resonant energy transfer rates between the isotopic species are studied by time resolved infrared emission. Such rates are important for isotope separation schemes using lasers. All of the rates are rapid. Direct comparisons can be made to theoretical models for resonant energy transfer processes.
By using multiphoton ionization spectroscopy, hundreds of previously unobserved states have been identified in the alkaline earth atoms Ca, Sr and Ba. In this technique, pulsed, tunable dye lasers are focused into a pipe containing the atomic vapor. A simple wire probe, inserted into the hot vapor, readily detects ions produced as a result of the multiphoton excitation. Excitation spectra from various well-defined initial states are obtained by re-cording the ion signal as a function of laser wavelength. In a single laser experiment, one obtains the two-photon excitation spectrum from the 1S0 ground state of the alkaline earths, to the Rydberg series of bound even-parity 1S0, 1D2 and 3D2 states, most of which have not previously been identified by conventional spectroscopy. In a two-laser version of this experiment, we have made the first observation of Rydberg series of even-parity auto-onizing states. In a three laser variation the excitation spectrum from the first excited 3S1 levels (of Ca, Sr and Ba) reveals the 3P0 Rydberg series. Since transitions from the 1S0 ground state to these series are spin-forbidden, they have not previously been seen in absorption spectra.
Several techniques for time-resolved Raman spectroscopy covering timescales from 10-8 to 10-1 sec are discussed. These methods are used to obtain the Raman spectra of several intermediates in the photosynthetic cycle of bacteriorhodopsin. Using these spectra and their temporal dependences, attempts are made at determining where deprotonation occurs, and if there is a cis-trans isomerization step in the cycle. Extension of the technique to other systems and shorter timescales is discussed.
Studies of alkaline earth oxide and monohalide molecules using laser induced photolumi-nescence spectroscopy (Mg0), tunable laser excitation spectroscopy (CaCl), laser-laser double resonance spectroscopy (CaCl) and microwave-laser double resonance spectroscopy (Ba0) are discussed and summarized. The advantages of these laser techniques over conventional methods of obtaining and assigning optical spectra are emphasized.
The use of mixed molecular crystals at low temperatures as systems in which to study laser induced photochemistry is discussed. As specific examples the photochemistry of s-tetrazine and its dimethyl derivative is treated. These molecules have previously been used as guests in host molecular crystals to illustrate solid state laser isotope separation and photochemical hole burning. The results presented here indicate that the low temperature tetrazine photochemistry may involve the sequential absorption of two photons. With this in mind, we are able to suggest a useful method of finding other materials for isotope separation and photochemical hole burning.
Molecules that absorb 10.5-μm CO2 laser radiation are important as candidates for laser isotope separation and as possible saturable absorbers, passive mode lockers, and subjects for various non-linear optics experiments. Two such molecules are SF6 and OsO4, which have dense and complex absorption features that were not resolved until the development of tuna-ble diode lasers (resolution <10-5 cm-1). The spectra of these two compounds have been ex-amined in the region 940-970 cm-1 and will be discussed, with special attention to calibra-tion techniques. Full assignments and analyses of the bands have been carried out, isotope shifts measured, and the molecular constants determined. Application of these results to the analysis of non-linear optics experiments will be discussed.