Emergence of Charge-Transfer-to-Solvent Band in the Absorption Spectra of Hydrogen Halides on Ice Nanoparticles: Spectroscopic Evidence for Acidic Dissociation

Abstract

Extensive ab initio calculations complemented by a photodissociation experiment at 193 nm elucidate the nature of hydrogen halide molecules bound on free ice nanoparticles. Electronic absorption spectra of small water clusters (up to 5 water molecules) and water clusters doped with hydrogen fluoride, hydrogen chloride and hydrogen bromide were calculated. The spectra were modeled at the time-dependent density functional (TDDFT) level of theory with the BHandHLYP functional using the reflection principle. We observe the emergence of a charge-transfer-to-solvent (CTTS) band in the absorption spectra upon the acidic dissociation of the hydrogen halides. The CTTS band provides a spectroscopically observable feature for the acidic dissociation. The calculated spectra were compared with our new experimental photodissociation data for larger water clusters doped with HCl and HBr. We conclude that HCl and HBr dissociate to a large extent on the surface of ice nanoparticles at temperatures near 120 K and photoactive products are formed. The acidic dissociation of HX leads to an enhancement by about 4 orders of magnitude of the HCl photolysis rate in the 200-300 nm region, which is potentially relevant for the halogen budget in the atmosphere.

Velocity map imaging of HBr photodissociation in large rare gas clusters

“…[8][9][10][11][12][13][14] When this experiment was moved to our laboratory in Prague, the studies have been extended to other systems, e.g., hydrogen halides on ice nanoparticles [15][16][17] or clusters of biologically relevant molecules. For instance, photodissociation of hydrogen halides on/in various large rare gas clusters HX·Rg n (n ∼ 10 2 atoms) was studied on a versatile cluster beam apparatus in Göttingen.…”

Atmospheric processes on ice nanoparticles in molecular beams

“…Figure 2C shows that the HCl molecules on the water cluster do not coagulate to HCl clusters as opposed to the Ar case shown in Figure 2A. An example of the H-fragment KED measured in several our studies (Poterya et al, 2007, 2008a; 2011; Ončák et al, 2008, 2011) is shown in Figure 2D. It exhibits some differences compared to the rare gas case: the direct exit fast fragments are missing entirely as well as the sharp peak at zero kinetic energy (cage effect); rather slow fragments extending to ≈0.5 eV occur with a broad maximum below 0.1 eV.…”

“…Ončák et al (2008, 2011) and Poterya et al, (2011) argued that the acidic dissociation in the ground state leads to a significant red-shift of the absorption spectra of the X − H 3 O + (H 2 O) n −1 species with possible atmospheric consequences.…”

“…Subsequently, Cl 2 is photolyzed to release Cl radicals into the stratosphere. However, the above mentioned red shift of the absorption spectra when HCl is adsorbed on the ice particles and acidically dissociated can significantly enhance the direct production of Cl radicals from photodissociation on ice particles (Ončák et al, 2008). Ončák et al, (2011) have outlined some questions which should be addressed in the future, namely: Should these direct processes on ice particles be included in the stratospheric ozone depletion models?…”

“…The details of ice nanoparticles generation in supersonic expansions has been recently investigated by Kim et al (2004), Manka et al (2012), and Li et al (2013). The individual particles can be investigated under controlled conditions in vacuum by various means: e.g., ionization (electron, photon) and mass spectrometry (MacTaylor and Castleman, 2000; Lengyel et al, 2012b); infrared (IR) spectroscopy (Yacovitch et al, 2011; 2012; Preston et al, 2012; Fujii and Mizuse, 2013) or ultraviolet (UV) photodissociation experiments (Kreher et al 1999; Li and Huber, 2001; Poterya, et al 2007, 2008a; 2011; Ončák et al, 2008, 2011); particle (electron, photon, neutron) scattering (Heath et al, 2002; Kim et al, 2004; Manka et al, 2012); special methods such as sodium doping and subsequent spectroscopies (Bobbert et al, 2002; Yoder et al, 2011; Pradzynski et al, 2012). Such experiments provide unprecedented molecular-level insight into the small particle generation, their (photo)chemistry and (photo)physics and detailed dynamics of the processes on/in these particles.…”