Dissociative desorption from CH3I by ultraviolet-laser radiation

Abstract

CH3 I molecules condensed in thick layers are exposed to pulsed UV-laser radiation at a wavelength of 266 nm. Atoms and molecules emitted from the surface are detected after a flight path by species and state selective resonant multiphoton ionization combined with a time-of-light mass spectrometer. This detection method is tested by photodissociation experiments of CH3 I in the gas phase. The two dissociation channels known from gas phase dissociation, yielding I(2 P3/2 ) and I*(2 P1/2 ) are identified in the velocity distribution of CH3 fragments desorbed from the surface. At low UV fluences this ‘‘direct’’ dissociative photodesorption dominates. Its similarity with gas phase photodissociation is studied in some detail. At UV fluences above 1.5 mJ/cm2 a strong increase of the overall desorption yield is observed and the composition of desorbed species changes. The process becomes similar to UV-laser photoablation of polymers.

Photolysis of ClNO adsorbed on MgO(100)

“…Thus, their translational and rotational energies are quenched and are quite different from the products of gas phase photolysis. Similar TOF spectra were also seen with CH31 on LiF (16) and HBr islands on LiF(001) (1 1) and interpreted in terms of products formed in lower layers which undergo many collisions before escaping. In the case of HBr, the H-atom TOF low energy tail increased with surface sites.…”

“…This makes them better candidates for studying processes driven by adsorbate excitation. The direct photolysis of adsorbates on insulator surfaces has been seen in the following systems: CH3Br (5-8), H2S (6,9), HBr (lo, 1 1), HC1 ( l l ) , OCS (12), C2H3Cl (13a), and NO2 (136) on LiF(001), CH212 on A1203 (14), Fe(CO)5 on A1203 (15), thick polycrystalline layers of CH31 on LiF(100) (16), C6H51 on LiF (17), CH31 and CD31 on MgO(100) (18) and NO2 on A120,(1 120) (19). In addition, the photodissociation of HBr (20) and CH3Br (21) on LiF(OOl), IBr (22) and ICI (23) on Mg0(001), and CH31 on MgO(100) (18c) have been examined theoretically.…”

“…Different mechanisms for parent desorption were proposed: excitation of color centers leading to shock waves which cause desorption (5,6,8,9,12,13), thermal heating (7), substrate quenching of excited adsorbates followed by desorption (17), explosive desorption (14,16), and laser annealing where molecules in unstable sites desorb (15).…”

“…molecules on the surface. Several groups have used resonance enhanced multiphoton ionization (REMPI) to detect reaction products (16)(17)(18)(19). This has enabled photodissociation products to be detected unambiguously and state-selectively and, in certain cases, has enabled comparisons to be made with vibrational state distributions obtained from the photolysis of gas phase molecules (16,18).…”

Surface pump-probe femtosecond-laser mass spectrometry: Time-, mass-, and velocity-resolved detection of surface reaction dynamics

“…Methyl iodide has been used as representative system for polyatomic photodissociation in numerous theoretical and experimental investigations. 19,20 In our experiment, MgO͑100͒ ultrathin films have been employed as a substrate for methyl iodide adsorption. 10 and 11 and references therein͒.…”