ALBERT

Notes: We present a theoretical and experimental investigation of the emission spectrum of dissociating water after excitation in the second absorption band (X˜ 1A1→B˜ 1A1). The calculations are performed in the time-dependent wave packet formalism employing an ab initio potential energy surface. All three degrees of freedom (the two OH stretching modes and the HOH bending mode) are taken into account. The B˜ 1A1 potential energy surface depends strongly on the HOH bending angle which leads to very fast opening of this angle after the water molecule is promoted to the excited electronic state. As a consequence, we observe, both experimentally and theoretically, the excitation of high bending states in the X˜ ground state. According to the wave packet study the emission spectrum is determined in the first ten femtoseconds of the motion in the excited state. The agreement with the measured spectrum for an excitation wavelength of 141.2 nm is good.

Notes: The photodissociation of H2S through excitation in the first absorption band (λ≈195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD-CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A‘ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we treat the coupling between the two electronic states in a diabatic representation extracting the coordinate-dependent mixing angle from the CI coefficients of the electronic wave functions. The nuclear motion is treated in three dimensions in an exact quantum mechanical approach by propagation of a two-component time-dependent wave packet. The calculated absorption spectra for H2S and D2S satisfactorily agree with the measured spectra. In particular, the calculations reproduce the diffuse structures with energy spacing of about 1200 and 850 cm−1 for H2S and D2S, respectively. Furthermore, the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. The photodissociation of H2S is a prototype for very fast electronic predissociation. The photon preferentially excites the binding (diabatic) state. This state, however, is quickly depleted by strong coupling to the dissociative (diabatic) state with the complex finally breaking up into products H and HS. The electronic quenching takes place on the time scale of one internal vibrational period only.Our calculations unambiguously confirm that the diffuse structures superimposed to the broad background are caused by symmetric stretch motion—in the binding state—and not by activity in the bending mode as originally assumed.

Notes: The spectroscopic parameters of Er3+-doped crystals were determined with regard to the upconversion laser parameters of the green transition 4S3/2→4I15/2. The influence of excited-state absorption on this laser channel was determined. Furthermore, upconversion pump mechanisms using ground-state and excited-state absorption around 810 and 970 nm were investigated by direct measurements of excited-state absorption. The spectroscopic results confirm the pulsed room-temperature laser experiments on the 4S3/2→4I15/2 transition. The lasers based on Er:LiYF4, Er:Y3Al5O12, and Er:Lu3Al5O12 were directly excited into the upper laser level by an excimer laser pumped dye laser in the blue spectral range. In Er:LiYF4, Er:KYF4, and Er:Y3Al5O12, laser action was achieved with two-step upconversion pumping by a Ti:sapphire laser and a krypton ion laser. In the case of the fluorides, the additional pumping with the krypton ion laser was not necessary. The laser emission wavelengths were 551 nm for Er:LiYF4, 561 nm for Er:Y3Al5O12 and Er:Lu3Al5O12, and 562 nm for Er:KYF4. In addition, green quasi-cw laser emission of Er:LiYF4 pumped with an argon-ion laser was realized at room temperature.

Notes: cw lasing at 1.6 μm was obtained for the first time in Cr, Yb, Er:fluoroaluminate glass. Double step pumping via Cr3+ and Yb3+ with a krypton laser yields a threshold pump power of 80 mW. Efficient lasing can be expected using glass samples of optimized dopant concentration and improved optical quality.

Notes: We report room-temperature pulsed up-conversion laser oscillation in Er-doped LiYF4 and KYF4 at 551 and 562 nm, respectively. In both crystals laser oscillation is observed on the 4S3/2-4I15/2 ground state transition. Excitation was provided by a tunable flashlamp-pumped Ti:sapphire laser in the spectral region around 810 nm. Additional pumping with a continuous wave krypton ion laser at 647 nm was beneficial to both lasers. Laser action has also been observed in Er-doped Y3Al5O12 on the same transition.

Notes: We report the generation of ultrashort light pulses from cw-pumped Cr, Tm:YAG and Cr, Tm, Ho:YAG lasers actively mode-locked by an acousto-optic modulator. The pulse duration obtained from Tm:YAG was 45 ps which was approximately two times the bandwidth limit. Mode-locked pulses less than 800 ps were also produced in Cr, Tm, Ho:YAG.

Notes: We report room-temperature upconversion pumped continuous wave laser emission of 1% Er3+:LiYF4 at 551 nm excited by a Ti:sapphire laser at 810 nm. Output powers of up to 40 mW with output coupling of 6.6% have been obtained by using nearly concentric resonator design.

Notes: We report the first demonstration of a green erbium-doped crystal laser at room temperature. Excited with a pulsed dye laser, the erbium-doped LiYF4 crystal emitted on the 4S3/2–4I15/2 ground state transition. As expected from fluorescence measurements, the emitted radiation was polarized parallel to the optical axis. Laser oscillation has also been observed in erbium-doped Y3Al5O12 and Lu3Al5O12 crystals.

Notes: We report efficient room-temperature continuous-wave intracavity frequency doubling of an upconversion-pumped Er(1%):YLiF4 laser at 850 nm. A titanium–sapphire laser was used for excitation of the 4S3/2→4I13/2 transition in erbium. The maximum laser output power at 850 nm was 1200 mW. Intracavity frequency doubling the fundamental wave utilizing lithium triborate as nonlinear crystal yielded a maximum second-harmonic output power of 540 mW at 425 nm. © 1998 American Institute of Physics.

Notes: Double-differential cross sections for the interaction of Na(3 2S) and Na(3 2P) with SF6 have been measured in crossed beam experiments for center of mass collision energies between 0.25 and 1.75 eV. In comparison with recently reported experiments the reaction with vibrationally excited SF6 is found to be more effective than the one with electronically excited Na. Results from an ab initio CASSCF calculation with Na in the ground state and the 3P state are presented. The experimental findings and the results from the calculation lead us to two different models for the reaction in the ground state and the excited state: While the well known harpooning model is verified for the ground state the reactive collisions with excited Na are mediated by nonadiabatic (nonreactive) transitions to the ground state surface. For these transitions the vibrational motion of SF6 is much more efficient than the relative motion in the collision. © 1996 American Institute of Physics.