New experimental results and theoretical calculations are reported for the optical electron transfer (ET) and subsequent reverse radiationless ET for the molecules (NH3)5RuIIINCRuII(CN)−5 and (NH3)5FeIIINCRuII(CN)−5. A procedure is presented for extracting many of the key parameters in ET theory from a combined analysis of resonance Raman data and the optical absorption ET band shape. Using these parameters, ET rates have been calculated using contemporary ET models. The experimental and theoretical rates agree within the uncertainty of the theoretical predictions, which results from an uncertainty in some of the parameters. The results demonstrate that inertial solute/solvent interactions and intramolecular sources of fast energy fluctuations play an important role in ultrafast ET kinetics for these compounds.

We report a new approach to obtain single-transverse-mode operation of a multimode fiber amplifier, in which the gain fiber is coiled to induce significant bend loss for all but the lowestorder mode. We have demonstrated this method by constructing a coiled amplifier using Ybdoped, double-clad fiber with a core diameter of 25 ym and NA of -0

The dissociation, internal conversion, and vibrational relaxation of photoexcited I2− in ethanol have been examined using ultrafast transient-absorption spectroscopy. I2− was photoexcited at 770 nm (1.6 eV) and probed on the subpicosecond time scale at 15 wavelengths between 580 and 950 nm, permitting a determination of the temporal evolution of the absorption spectrum. The data reveal that internal conversion and vibrational relaxation at the top of the well are extremely rapid (≤0.3 ps), with loss of the final 0.3 eV of energy (v≤20) occurring on a time scale of ∼4 ps. Simple kinetic and spectral models are able to qualitatively account for the observed behavior of the transient-absorption signals.

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We have measured the nascent HD(v′=1, j′) product rotational distribution from the reaction D+H2(v, j) in which the H2 reagent was either thermal (v=0, j) or prepared in the level (v=1, j=1) by stimulated Raman pumping. Translationally hot D atoms were obtained by uv laser photolysis of DBr or DI. Photolysis of DBr generated D atoms with center-of-mass collision energies (Erel) of 1.04 and 0.82 eV, which corresponded to the production of ground state Br and spin–orbit-excited Br*, respectively. The Erel values for DI photolysis were 1.38 and 0.92 eV. Quantum-state-specific detection of HD was accomplished via (2+1) resonance-enhanced multiphoton ionization and time-of-flight mass spectrometry. Vibrational excitation of the H2 reagent results in substantial rotational excitation of the HD(v′=1) product and increases the reaction rate into v′=1 by about a factor of 4. Although the quantum-mechanical calculation of Blais et al. [Chem. Phys. Lett. 166, 11 (1990)] for the D+H2(v=1, j=1)→HD(v′=1, j′)+H product rotational distribution at Erel=1.02 eV is in qualitative agreement with experiment, it does not quantitatively agree with the measured distribution. Specifically, the calculated distribution is too hot by 2–3 rotational quanta, and the predicted enhancement in the v′=1 rate with reagent vibrational excitation is too large by 67%±9.

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Yb-doped fibers are widely used in laser applications requiring high average output powers and high-peak-power pulse amplification. Photodarkening (PD) is recognized as one limiting factor in these fibers when pumped with high-intensity radiation. We describe an approach for performing quantitative PD studies of fibers, and we present measurements of the rate of PD in Yb-doped single-mode fibers with varying inversion levels. The method is applicable to large-mode-area fibers. We observed a seventh-order dependence of the PD rate on the excited-state Yb concentration for two different fibers; this result implies that PD of a Yb-doped fiber source fabricated using a particular fiber will be strongly dependent on the configuration of the device.

We have studied rotational energy transfer (RET) in collisions of OH with the bath gases Ar, N2, O2, and H2O at 293 K. Rotationally hot OH(X 2Π3/2, v″=0, N″=1–12) was generated by photolysis of H2O2 at 266 nm, and collisional relaxation of the nascent rotational distribution was monitored by laser-induced fluorescence. The data are remarkably well described by an exponential-gap model for the matrix of state-to-state RET rate constants. For Ar, N2, and O2, RET rates are significantly faster at low N″ than high N″; for H2O, RET is approximately an order of magnitude faster than for the other bath gases, and the rate is not as strongly dependent on N″. The rates of rotationally inelastic energy transfer are similar in the X and A states, but the X-state depopulation rate constants (including nearly elastic, Λ-doublet-changing collisions) are faster than the A-state values. By comparing the depopulation rates derived from the present experiment with previous linewidth measurements, we conclude that RET is the dominant source of pressure broadening for OH microwave transitions and makes a significant contribution for ultraviolet A–X transitions. While generally good agreement is found between the present results and previous OH RET studies for both the ground and excited electronic states, some significant discrepancies are noted.