J6 Laser alchemy: The new chemistry

J6 Laser alchemy: The new chemistry

Volume 18, n u m b e r 1 J6 Of'TICS COMMUNICATIONS L A S E R ALCHEMY: THE NEW CHEMISTRY (Invited). R.N. Z A R E Physics Department, Columbia Univer...

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Volume 18, n u m b e r 1 J6

Of'TICS COMMUNICATIONS

L A S E R ALCHEMY: THE NEW CHEMISTRY (Invited). R.N. Z A R E

Physics Department, Columbia University, New York, New York 10027, USA A review is presented of recent applications of lasers to various chemical problems. A m o n g topics covered include: (1) the laser detection and laser control of chemical reactions; (2) the direct observation in molecular b e a m s of radiationless processes and organic rearrangement reactions; (3) laser separation of isotopes by photochemical m e a n s using scavengers; and (4) the laser generation of transient diffusion photocurrents in liquids without the application of an external electric field.

J7

ABSORPTION, EMISSION AND CHEMICAL CHANGES OF H Y D R O G E N F L U O R I D E (HF) U N D E R HIGH POWER H F - L A S E R I R R A D I A T I O N H. PUMMER, D. PROCH, U. SCHMAILZL and K.L. KOMPA

Max-Planck.lnstitut fiir Plasmaphysik, EuratomAssociation, D-8046 Garching, Fed. Rep. Germany As one technique in laser induced chemistry an infrared laser is used to excite the lowest vibrational states o f one of the reaction partners, thus enhancing the rate of reaction. The anharmonicity of the vibration normally prohibits the direct optical p u m p i n g of higher vibrational states with the same laser wavelength. In order to investigate the chemical influence of high vibrational excitation, a way has to be found to prepare molecules in the corresponding q u a n t u m states. In this paper we present experiments and a c o m p u t e r model which deal with the generation o f highly vibrationally excited h y d r o g e n fluoride via laser absorption followed by fast vibrational-vibrational energy exchange. This fast V - V process which is partially decoupled from the other degrees of freedom causes the molecules to climb up the "vibrational ladder" in a way first proposed by Treaner et al. The computer model uses the rate equation approximation for the first 20 vibrational levels, level 20 corresponding to the dissociation c o n t i n u u m . The laser pulses used in the calculations and e x p e r i m e n t s are in the range of 0.1 to 100 MW/cm 2 with r f w h m = 200 nsec and consist of 14 lines in the P branch of the V = 4 - 3 , 3 - 2 , 2 - 1 , and 1 - 0 bands. Gas pressures are several 10 torr to guarantee an appreciable collisional energy exchange during the laser pulse. This model predicts a strong intensity-dependent absorption which increases with increasing pulse intensities, inversion for some high rotational states of the V = 3 - 2 , 2 - 1 , 1 - 0 bands and considerable dissociation of the HF molecules. These three points were verified experimentally. Compared to the necessary simplifications made in the c o m p u t e r model, the agreement between theory and experiment is remarkably good. The results can be understood in terms of an energy reser-

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voir formed by highly vibrationally excited molecules. At. cording to the theory these molecules are extremely et'ficie~t in removing vibrational energy out of the optically p u m p e d states by fast V - V collisions, in the same time putting the less excited molecule to a state from which it can absorb again. A build-up of such a reservoir can thus cnhance dissociation and absorption in a self-accelerating wa}. The increase of absorption with increasing pulse intensities which cannot be caused by cascade effects can be explained by this inodel. We would like to stress two points: the possibility to excite molecules up to the dissociative levels in the :~bove mentioned way has been shown. On the other hand, a build-up of a reserw)ir containing an appreciable n u m b e r of highly excited molecules causes very fast energy removal out of low vibrational states. In high power ttF lasers operating at some 100 torr this could be one of the major loss m e c h a n i s m s which limit the energy extraction.

J8

PHOTODISSOCIATION AND EXCITED STATES OF CO~ J.T. MOSELEY, P.C. COSBY, J.H. LING and J.R. PETERSON

Molecular Physics Center, S.R.L, Menlo Park, California 94025, USA The photodissociation cross section CO~ + h v -~ O- + CO 2 has been measured using a drift tube mass spectrometer [1,2], a continuously tunable dye laser, and an argon laser over the wavelength range from 6950 to 4579 A. In the experimental apparatus the CO~ ions are formed by i o n - m o l e c u l e reactions in CO2, and undergo a large n u m b e r of thermatizing collisions before they are intersected by the chooped intracavity p h o t o n beam, sampled, mass-selected and individually detected. By counting for alternate periods when the laser is on and off, the cross section for the total loss of parent ions, the p h o t o d e s t r u c t i o n cross section, can be determined. The photodissociation cross section can be determined by similar observation of p r o d u c t p h o t o f r a g m e n t ions. For C O ) over the wavelength range considered here the only photodestruction process is photodissociation into O- + CO 2. The photodissociation cross section obtained between 6950 A and 5270 A, using a continuously tunable dye laser, shows very detailed structure similar to that of an absorption or emission spectrum. The results between 6400 A and 5650 A, and for the discrete argon laser lines have been published previously [ 1,3 ]. Tests have shown that the parent CO~ ions were essentially in thermal equilibrium at 300 K, and that two-photon photodissociation or collisional dissociation following p h o t o n excitation to a weakly b o u n d excited state did n o t occur. We therefore conclude that the structure in the CO~ photodissociation cross section is characteristic o f an electronic excited state of the ion, which spontaneously dissociates into O- + CO 2. Using these results along with other supplementary meas-