The lowest excited singlet states of 4-(dimethylamino)-4'-cyanostilbene (DCS), 4-(dimethylamino)-4'-nitrostilbene (DANS), and 4-di(hydroxy-ethyl)amino-4'-nitrostilbene (DANS-diol) have been investigated by high-resolution fluorescence excitation and dispersed emission spectroscopy on samples seeded in supersonic expansions. Using ab initio calculations of the harmonic force fields of the electronic ground state as a starting point for the analysis of vibrational modes and frequencies, detailed vibrational assignments of the rich line structure of the S-1<--S-0 transition in these three push-pull stilbenes are reported. From the experimental and theoretical analysis, it becomes apparent that the excitation spectra are dominated by low-frequency vibrational modes of the stilbene-like backbone, albeit that a characterization in terms of trans-stilbene vibrational terminology is not very appropriate. The fluorescence excitation and emission spectra show significant Franck-Condon factors for transitions involving these vibrational modes, in line with significant geometry changes in the stilbene moiety upon electronic excitation. Comparison of the spectroscopic data of the DANS chromophore in DANS and DANS-diol demonstrates that the inclusion of the diol auxiliary function has a significant effect on its spectroscopic properties. Similarly, the comparison between solution and gas-phase data leads to the conclusion that spectroscopic data obtained in solution for DCS allow for a reasonably good extrapolation to the isolated molecule but that the structure of DANS is, even in low-polarity solvents, already modified to a larger extent than would a priori be expected.

Donor-acceptor compounds containing a phenylenediamine electron donor and a naphthalene, a cyanobenzene, or a cyanonaphthalene acceptor were studied. The two chromophores are connected by three different bridging units, consisting of CH2 groups linked to a semiflexible piperidine or piperazine ring or to a rigid 2,5-diazabicyclo[2.2.1]heptane group. All donor-acceptor compounds show photoinduced charge separation, resulting in the formation of a compact charge-transfer (CCT) state in nonpolar solvents. The conformational change needed to arrive at this species is impossible in a nonpolar polymer matrix and also in methylcyclohexane at low temperature. In the cases of the piperidine- and piperazine-bridged donor-acceptor compounds, evidence for the involvement of an extended charge-transfer (ECT) species as a precursor of the CCT species was obtained from time-resolved fluorescence measurements. In contrast to harpooning systems studied previously, the energy differences between the species in the present case are so small that their interconversions are reversible within the excited state lifetimes.

The lowest excited singlet states of the structurally rigid amines 1-azaadamantane and 1-azabicyclo[2.2.2]octane have been investigated by using fluorescence excitation spectroscopy on samples seeded in supersonic expansions. Based upon the notion that in both species the lowest excited singlet state is a Rydberg state with the ground state of the radical cation as its ionic core, excitation spectra have been analyzed by employing density functional calculations of the equilibrium geometries and force fields of the ground state of the neutral species and its radical cation. A good agreement is obtained between experimentally observed and theoretically predicted frequencies and intensities of vibronic transitions. Subsequent refinements of the geometry of the lowest excited singlet state are shown to account adequately for the minor differences between experiment and the computational results obtained by using the radical cation as a model for the lowest excited singlet state. From our analysis it also becomes apparent that the excited state is in both molecules subject to vibronic coupling with higher-lying excited states, as exemplified by the presence of transitions to non-totally symmetric vibrational levels. The results of the present study enable the determination of mode-specific reorganization energies accompanying ionization of 1-azaadamantane, which are shown to correspond qualitatively well with those determined in resonance Raman studies on the charge transfer transition in the electron donor-acceptor system 1, which contains 1-azaadamantane as the electron donor unit.

The lowest excited singlet states of the structurally rigid amines 1-azaadamantane and 1-azabicyclo[2.2.2]-octane have been investigated by using fluorescence excitation spectroscopy on samples seeded in supersonic expansions. Based upon the notion that in both species the lowest excited singlet state is a Rydberg state with the ground state of the radical cation as its ionic core, excitation spectra have been analyzed by employing density functional calculations of the equilibrium geometries and force fields of the ground state of the neutral species and its radical cation. A good agreement is obtained between experimentally observed and theoretically predicted frequencies and intensities of vibronic transitions. Subsequent refinements of the geometry of the lowest excited singlet state are shown to account adequately for the minor differences between experiment and the computational results obtained by using the radical cation as a model for the lowest excited singlet state. From our analysis it also becomes apparent that the excited state is in both molecules subject to vibronic coupling with higher-lying excited states, as exemplified by the presence of transitions to non-totally symmetric vibrational levels. The results of the present study enable the determination of mode-specific reorganization energies accompanying ionization of 1-azaadamantane, which are shown to correspond qualitatively well with those determined in resonance Raman studies on the charge transfer transition in the electron donor-acceptor system 1, which contains 1-azaadamantane as the electron donor unit.

Titaniumdiboride (TiB2) is an attractive candidate for wear- and corrosion-resistant coatings. The photothermal deposition of TiB2 by hydrogen reduction of TiCl4 and BCl3 has been studied. In contrast to thermal chemical vapor deposition (CVD), the laser-induced chemical vapor deposition (LCVD) results in a surface reaction rate limited process. High deposition growth rates are obtained and powder production is prevented. The enhancement of the growth rate by UV light from an ArF excimer laser is larger than 40%.

: This patent discloses an optically pumped semiconductor ring laser having a plurality of reflective elements optically aligned with one another to form a ring-shaped resonant cavity. A semiconductor lasing medium is mounted within the ring-shaped resonant cavity by a transparent, heat conductive mount located within a vacuum/cooling chamber of the type which allows the passage of a laser beam therethrough. A pump beam initiates a lasing action within the ring-shaped resonant cavity to produce said laser beam and said laser beam exits the resonant cavity as a pair of outputs. An alternate embodiment of the above described semiconductor ring laser provides a semblance of unidirectional lasing operation.

The time response of the classical photomultipliers 56DUVP and XP2020 in time-correlated single-photon counting has been measured at several wavelengths between 257 and 630 nm with a best result of 235 ps FWHM. Also, the time response has been measured of the microchannel-plate photomultiplier R1564U-01 and the crossed-field photomultiplier VPM-154M at the two wavelengths 273 and 546 nm. The best result was 47 ps at λ=273 nm. The use of a fast constant-fraction discriminator was crucial. The origin of after pulses in the time response curve of photomultipliers is discussed.

It is shown that upon pulse excitation by the frequency shift method, the nutation effect can also be observed in the changes of the spontaneous emissi

This paper reports the first direct observation of intramolecular vibrational redistribution (IVR) where two consecutive steps have been distinguished. The initially excited vibronic state ∼(6b2) of the van der Waals complex of s-tetrazine and argon changes into the intermediate state ∼(16a116b1) from which subsequently the final state ∼(16a2) originates. The temporal intensity profiles of selectively detected emission from each of these states of the complex have been measured on a picosecond time scale.

A Raman echo on a picosecond timescale on the υ = 0 → 1 vibrational transition in nitrogen gas (8 atm. ≈ 300 K) was observed. We have used two synchronously pumped dye lasers which were amplified in dye amplifiers. Using the ps pulses from these two dye lasers and the second harmonic from a Nd:YAG laser in a four wave mixing experiment the Raman susceptibility decay (analogous to the free induction decay) and the echo decay were measured.

We report tunable cw laser action of platelet semiconductors in both mode-locked and unmode-locked configurations in external cavities. Various cavity configurations including rings will be discussed The gain media are platelets of CdSSe, InGaAsP, and HgCdTe cooled and longitudinally pumped by argon, krypton, and Nd:YAG lasers.

We report the first optically pumped semiconductor laser operating in a ring cavity configuration. Output powers of more than 40 mW have been achieved from a CdS platelet optically pumped by an Ar+ laser at 476 nm. Tunability of the laser output was demonstrated from 493–502 nm using an intracavity prism and temperature tuning.

A sync-pumped cw dye laser system has been used to produce subpicosecond pulses. Pulses as short as 0.7 ps, assuming a single-sided exponential pulse shape, were observed. A set of experiments was performed to investigate the origin and effects of noise in the sync-pumped system. A digital and an analog feedback loop have been designed to optimize the pulse width. The noise has been lowered by 10 dB for frequencies up to 10 kHz; long-term drift is also controlled by this method. A four-stage dye laser amplifier, pumped by a Nd:YAG laser which operates at a 10-Hz repetition rate, is synchronized electronically to the dye-laser picosecond pulses. A gain of 3×106 has been achieved.

To assess the role of epidermal melanin in a patient with porphyria variegata and vitiligo, the MED was determined in pigmented and vitiliginous skin for wavelengths of 310, 405 and 500 nm. The energy required to elicit erythema by irradiating vitiliginous skin at 310 nm was half that for pigmented skin. For 405 nm the differences was 4-fold and at 500 nm 2-fold. A possible explanation for the different protection by melanin against light of 310, 405 and 500 nm is given. UV-B irradiation, as a potent stimulus for melanization of the skin, is proposed as an additive measure in the protection against photosensitivity reactions in porphyria patients.

We report the first tunable laser action of a platelet semiconductor in an external cavity. We have achieved cw output powers of up to 9 mW from a CdS platelet optically pumped by an Ar+ laser at 476 nm. The energy conversion efficiency is 10% into the TEM00 mode. Tunability of the laser output was demonstrated from 495 to 501 nm using an intracavity prism with the crystal kept at a temperature of 95 K. Additional tuning to 504 nm was achieved by varying the temperature to 140 K.