Laser Pump-probe Technique Research Articles

Dynamics of vibrational wave packets in the C 1Π u state of Cs 2

The dynamics of vibrational wave packets in the weakly predissociated C 1Π u of Cs 2 have been examined by two color, femtosecond laser pump—probe techniques. The motion of the wave packet is detected by photoionizing the dimer with a time-delayed 620nm pulse and monitoring the time-and energy-integrated photoelectron signal with a proportional counter. Since the anharmonicity of Cs 2(C 1Π u is light (ω e/ω e x e≈700) and the ground state vibrational manifold is equilibrated at T≈500 K, wave packets consisting of the superposition of wavefunctions of more than 20 vibrational levels are formed that dephase in ≈ 18 ps.

Nonradiative relaxation and ionization of the F center in NaBr studied with picosecond optical pulses.

With a picosecond pump-probe laser technique the recovery of the ground-state population after optical excitation of the F center in NaBr is studied. Two decay components are observed, the fastest one dominating at low temperatures. Its time constant is interpreted as the lifetime of the relaxed excited state and is established to be 61 ns at 10 K. This value is perfectly consistent with the experimental emission efficiency and the expected radiative lifetime, provided that the nonradiative transition to the ground state is assumed to occur after the relaxed excited state is reached. Within the same assumptions a nonradiative lifetime of 27 ns at zero temperature is predicted for the F center in NaI. A theoretical expression for vibronic tunneling to the ground state from a thermalized excited state is applied to the F center in alkali halides and is shown to agree equally well with the experimental emission efficiencies as the Dexter-Klick-Russell criterion. The same expression also accounts for the strong temperature dependence of the nonradiative relaxation process observed in NaBr below 100 K. At higher temperatures an accurate analysis of the radiationless transition rate is encumbered by the influence of ionization on the lifetime of the excited state. The relative contribution of the second, much slower decay component increases rapidly above 70 K and is related to retrapping of electrons released by ionization of F and F centers. © 1992 The American Physical Society.

Nascent rotational quantum state distribution of NaH (NaD) from the reaction of Na*(4 2P) with H2, D2, and HD

The nascent rotational quantum state distributions of NaH and NaD products resulting from the reactions of Na*(4 2Pj) with H2, D2, and HD have been determined using the laser pump–probe technique. We have observed a bimodal rotational distribution with a minor component peaking at low J and a major component peaking at high J. We have observed no evidence for a kinematic isotope effect on the product distribution. Our results are consistent with a model wherein the reaction occurs predominantly on the attractive 2B2 potential energy surface in near C2v geometry with the rotational distribution being determined late in the exit channel.

Laser Picosecond Acoustics in Various Types of Thin Film

We describe the excitation and detection of picosecond stress pulses in a variety of thin films using an ultra fast laser pump-probe technique to measure the film reflectance variation. An opaque thin film of amorphous diamond-like carbon of known thickness is first investigated and the longitudinal sound velocity derived. Two methods of testing transparent films on opaque substrates are then considered, one a conventional technique in which an opaque transducer layer is deposited on the transparent film, and one a new technique in which the transparent film is uncoated. For the latter technique it is shown how both the sound velocity and the film thickness can be determined.

Picosecond absorption dynamics of photoexcited InGaP epitaxial films

The absorption recovery of a photoexcited InGaP epitaxial film 0.4 μm thick was investigated using the pump-probe laser technique and found to have a time constant of 55 ps at room temperature. Measurements done in the temperature range of 300–50 K show the decay of the photoexcited carrier distribution to be dominated by ambipolar diffusion and surface recombination. The measured absorption recovery time constant corresponds to an ambipolar diffusion coefficient D≳2.8 cm2/s and a surface recombination velocity of S≳4×105 cm/s at room temperature.

Picosecond studies on the photodissociation dynamics of aromatic endoperoxides

Studies on various aromatic endoperoxides (POs) reveal a different photodynamic behavior despite similar excitation conditions and despite similar chromophore structures. Using picosecond laser pump-probe technique it was found that heterocoerdianthrone (HECD), dissolved in dichloromethane, is produced from photocycloreversion of its endoperoxide with a time constant of τ=40±10 ps. Since the lifetime of the photoreactive S3 state is less than 3 ps, a two-step mechanism is expected. Photocleavage of the endoperoxide of anthradichromene (ADCPO) happens in 55±15 ps, whereas the endoperoxides of dimethylhomöocoerdianthrone (HOCDPO) and dimethoxyhomöocoerdianthrone (DMHDPO) photodissociate much faster. For their photodissociation we can state an upper time limit of 5 ps. The results of polarization and solvent dependent experiments demonstrate that the observed rise of signal is directly correlated with the formation of the parent compound in the ground state. The dynamics of the photodissociation of HECPO does not depend on polarity or viscosity of solvent. Therefore, an oxciplex configuration or an intermediate zwitterion cannot be involved in the photocycloreversion of aromatic endoperoxides. The model of a biradical mechanism must be claimed instead.