Molecular desorption from ionic molecular crystals following resonant UV excitation

Conference · Wed Dec 31 04:00:00 EST 1997

OSTI ID:560054

Hess, W P; Beck, K M; Taylor, D P; McCarthy, M I ^[1]

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1. Pacific Northwest National Lab., Richland, WA (United States)

We report the results of a photostimulated desorption study of neutral CO product from geologic calcite using 193 nm excimer laser radiation. The excitation laser flux is maintained at low levels (<0.3 MW/cm2) to produce desorption yields of no more than 0.01% of a monolayer per pulse. The low yields provide desorbed product species free of secondary interactions following fragment ejection from the crystal surface. Detailed product state distributions are obtained using (2+1) resonance enhanced multiphoton ionization. The CO translational energy can be characterized by a temperature (T = 115 K) significantly lower than that of the substrate (T = 295 K). The CO rotational state distribution can be characterized by a temperature of 295 K but displays reproducible non-Boltzmann features, i.e. a step structure every 4 or 5 rotational states. The CO vibrational distribution has a 6% contribution from v{double_prime}=1 yielding a temperature of 1100 K. The CO yield is linear with laser power, and displays no detectable threshold or incubation period with laser exposure. The low kinetic energy of the desorbed CO indicates that the photostimulated desorption process is efficient for a narrow range of energies and the nonthermal CO distributions implicate decay of a surface species or state.

• OSTI ID: 560054
• Report Number(s): CONF-970443--
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
58 GEOSCIENCES
CALCITE
CARBON MONOXIDE
DESORPTION
EXCITATION
IONIZATION
KINETIC ENERGY
LASER RADIATION
MOLECULAR CRYSTALS
ROTATIONAL STATES