Ultraviolet photochemistry and photophysics of weakly-bound (HI)2 clusters via high-n Rydberg time-of-flight spectroscopy

Abstract

The high-n Rydberg time-of-flight (HRTOF) technique has been used to obtain translational energy distributions of hydrogen atoms deriving from weakly-bound (HI)2 clusters photoexcited at 266 nm. A number of distinct features were observed and were used to establish much of the photophysics and photochemistry. Though the geometric structure of (HIh has not been determined experimentally, equilibrium geometries have been estimated by using several semiempirical theoretical methods, all of which predict an approximately 90" L-shaped structure with one hydrogen localized between the two iodine atoms (the interior hydrogen) and the other pointing outward (the exterior hydrogen). Zero-point amplitudes are expected to be large. The photolytic removal of the exterior hydrogen yields I-HI and I*-HI radical-molecule clusters whose properties can be described, at least qualitatively, by using the formalism put forth by Hutson and co-workers, who carried out detailed calculations for the analogous C1-k HCl system. Photodissociation of the HI moiety whose hydrogen is interior can also yield radical-molecule clusters, as well as initiate intracluster reactive and/or inelastic scattering processes. Photoproducts that contain the HI chromophore such as HI(vj), I-HI, and I*-HI can also be efficiently photoexcited, yielding hydrogen atoms having signatures that reflect their parentages. Peaks in the translational energy distribution corresponding to photodissociation of HI in v = 1 and 2 are identified and confirmed by H -D substitution. Furthermore, v = 0 rotational levels having 7 5 j 5 13 are just barely resolved. The most likely source of intemally excited HI is believed to be inelastic scattering in which the internal hydrogen strikes the adjacent HI. This is deduced from the theoretical work of Aker and Valentini, who employed the method of quasiclassical trajectories with a potential surface developed by Last and Baer and modified by Clary. These calculations suggest that the likely L-shaped geometry of (HI);? is compatible with inelastic scattering via a failed reaction mechanism, whereas the hydrogen exchange reaction has low probability since it favors near-linear H-IH approaches. Low-energy shoulders offset slightly from the monomer peaks are most likely due to inelastic and elastic scattering of the internal hydrogen as it leaves. The photolysis of I*-HI clusters can be identified by an inelastic process in which I* is deactivated, thereby yielding hydrogen atoms having translational energies in excess of the highest monomer peak by slightly less than the iodine spin-orbit splitting. Such a peak in the TOF spectrum is observed. It is inevitable that some 12 is formed with large interatom separation via the photolytic removal of hydrogen, which can occur by either H2 formation (hydrogen abstraction) or sequential photolysis.

Bimolecular reactions observed by femtosecond detachment to aligned transition states: Inelastic and reactive dynamics

“…15 The formation of a complex in the inelastic E→V process of HIϩI ͑from HI dimers͒ has been shown recently by detecting the translational energy distributions of H atoms. 25 This is consistent with the above picture ͑CH 3 IϩI͒ and suggests a similar fs study of the HI system in this laboratory. It is interesting to note that the lifetimes of excited-state complexes ͑Hg*•N 2 ͒ in vdW wells have been reported to be of similar magnitude ͑2-5 ps͒.…”

Study on the (HI) m (H2O) n Clusters by Photofragment Translational Spectroscopy and Ab-Initio Calculation

“…As the partial pressure of HI in our gas sample (total v.p. of hydroiodic acid ≈ 7.6 Torr) is much lower than 33 Torr HI in the sample used by Wittig et al (4.4% HI in He, 760 Torr) [21], for which they claimed that no (HI) 2 cluster found in their sample, therefore (HI) 2 cluster would be more difficult to be formed in our case. And (HI) m (m > 2) would be much more difficult to be formed.…”

“…Wittig's group [21] and Young [22] reported that there are several reaction channels for the photodissociation of (HI) 2 . Purnell et al [23] and Tisch et al [24] observed the Coulombic explosion of HI clusters when irradiated by intense femtosecond laser.…”

Photodissociation of (ICN)2 van der Waals dimer using velocity imaging technique

“…They could not be detected in photodissociation or bimolecular reactions of the monomer species. [4][5][6][7][8][9][10][11][12][13] Such new reaction channels are governed by the restricted approach geometries and impact parameters available to the reagents in the clusters.…”