Sharon C. Kettwich

The Cl + H2 → HCl + H Reaction Induced by IR + UV Irradiation of Cl2 in Solid para-H2: Experiment†

We report IR + UV coirradiation photolysis experiments conducted on Cl(2)-doped para-hydrogen (p-H(2)) crystals at 1.8 K, using pulsed 355 nm UV radiation and cw broad-band near-IR light from a FTIR tungsten source. The amount of HCl photoproduct is monitored using FTIR spectroscopy as a function of the IR + UV exposure time. Detailed analysis of the HCl growth kinetics reveals that the reaction Cl + H(2)(v=1,J=0) --> HCl + H is playing a significant (15%) role in the in situ photochemistry. In contrast, UV-only photolysis experiments conducted under similar conditions produce almost exclusively (99%) isolated Cl atom photofragments, indicating the reaction Cl + H(2)(v=0,J=0) --> HCl + H is not readily occurring. This combination of photolysis experiments confirms that under these conditions, the Cl + H(2) reaction probability increases by a factor greater than 25 for Cl atom reactions with H(2)(v=1) versus H(2)(v=0). These results are therefore consistent with the expectation that vibrational excitation of the H(2) reagent lowers the reaction threshold and increases the reaction cross section for the Cl + H(2) reaction. These experimental studies were motivated by and are compared to the quantum model simulations reported by Korolkov, Manz, and Schild in the accompanying paper.

Transient H2O Infrared Satellite Peaks Produced in UV Irradiated Formic Acid Doped Solid Parahydrogen.

We report newly identified satellite features of the R(0) rovibrational transition of all the fundamental modes of HDO and the ν3 mode of H2O measured via FTIR spectroscopy immediately after the 193 nm in situ photolysis of formic acid (HCOOH and DCOOD) in solid parahydrogen. The intensities of these satellite features decay slowly with a time constant of τ = 121(7) min after photolysis, even when the sample is maintained below 2 K. We propose that the van der Waals complex H···H2O (H···HDO) is the carrier of the satellite peaks and that these metastable complexes are produced after the low-temperature tunneling reaction of the OH (OD) photoproduct with the parahydrogen host.

Photodissociation of molecular bromine in solid H2 and D2: spectroscopy of the atomic bromine spin-orbit transition.

We report 355 nm photodissociation studies of molecular bromine (Br2) trapped in solid parahydrogen (pH2) and orthodeuterium (oD2). The product Br atoms are observed via the spin-orbit transition ((2)P(1/2)<-- (2)P(3/2)) of atomic bromine. The quantum yield (Phi) for Br atom photoproduction is measured to be 0.29(3) in pH2 and 0.24(2) in oD2, demonstrating that both quantum solids have minimal cage effects for Br2 photodissociation. The effective Br spin-orbit splitting increases when the Br atom is solvated in solid pH2 (+1.1%) and oD2 (+1.5%); these increases are interpreted as evidence that the solvation energy of the Br ground fine structure state ((2)P(3/2)) is significantly greater than the excited state ((2)P(1/2)). Molecular bromine induced H2 infrared absorptions are detected in the Q1(0) and S1(0) regions near 4150 and 4486 cm(-1), respectively, which allow the relative Br2 concentration to be monitored as a function of 355 nm photolysis.

Infrared studies of ortho-para conversion at Cl-atom and H-atom impurity centers in cryogenic solid hydrogen

We report infrared spectroscopic studies of H2 ortho-para (o/p) conversion in solid hydrogen doped with Cl-atoms at 2K while the Cl+H2 (v=1)→HCl+H infrared-induced chemical reaction is occurring. The Cl-atom doped hydrogen crystals are synthesized using 355nm in situ photodissociation of Cl2 precursor molecules. For hydrogen solids with high ortho-H2 fractional concentrations (Xo=0.55), the o/p conversion kinetics is dominated by Cl-atom catalyzed conversion with a catalyzed conversion rate constant Kcc=1.16(11)min−1 and the process is rate-limited by ortho-H2 quantum diffusion. For hydrogen crystals with low ortho-H2 concentrations (Xo=0.03), single-exponential decay of the ortho-H2 concentration with time is observed which is attributed to H-atom catalyzed o/p conversion by the H-atoms produced during the infrared-induced Cl+H2 reaction. The measured H-atom catalyzed o/p conversion kinetics indicates the H-atoms are mobile under these conditions in agreement with previous ESR measurements.

High-resolution infrared spectroscopy of atomic bromine in solid parahydrogen and orthodeuterium

This work extends our earlier investigation of the near-infrared absorption spectroscopy of atomic bromine (Br) trapped in solid parahydrogen (pH2) and orthodeuterium (oD2) [S. C. Kettwich, L. O. Paulson, P. L. Raston, and D. T. Anderson, J. Phys. Chem. A 112, 11153 (2008)]. We report new spectroscopic observations on a series of double transitions involving excitation of the weak Br-atom spin-orbit (SO) transition ((2)P(1/2) ← (2)P(3/2)) in concert with phonon, rotational, vibrational, and rovibrational excitation of the solid molecular hydrogen host. Further, we utilize the rapid vapor deposition technique to produce pH2 crystals with a non-equilibrium mixture of face centered cubic (fcc) and hexagonal closed packed (hcp) crystal domains in the freshly deposited solid. Gentle annealing (T = 4.3 K) of the pH2 sample irreversibly converts the higher energy fcc crystal domains to the slightly more stable hcp structure. We follow the extent of this conversion process using the intensity of the U1(0) transition of solid pH2 and correlate crystal structure changes with changes in the integrated intensity of Br-atom absorption features. Annealing the pH2 solid causes the integrated intensity of the zero-phonon Br SO transition to increase approximately 45% to a value that is 8 times larger than the gas phase value. We show that the magnitude of the increase is strongly correlated to the fraction of hcp crystal domains within the solid. Theoretical calculations presented in Paper II show that these intensity differences are caused by the different symmetries of single substitution sites for these two crystal structures. For fully annealed Br-atom doped pH2 solids, where the crystal structure is nearly pure hcp, the Br-atom SO transition sharpens considerably and shows evidence for resolved hyperfine structure.

Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Lin-(C2H4)m Complexes

Complexes of lithium atoms with ethylene have been identified as potential hydrogen storage materials. As a Li atom approaches an ethylene molecule, two distinct low-lying electronic states are established; one is the 2A1 electronic state (for C2v geometries) that is repulsive but supports a shallow van der Waals well and correlates with the Li 2s atomic state, and the second is a 2B2 electronic state that correlates with the Li 2p atomic orbital and is a strongly bound charge-transfer state. Only the 2B2 charge-transfer state would be advantageous for hydrogen storage because the strong electric dipole created in the Li-(C2H4) complex due to charge transfer can bind molecular hydrogen through dipole-induced dipole and dipole-quadrupole electrostatic interactions. Ab initio studies have produced conflicting results for which electronic state is the true ground state for the Li-(C2H4) complex. The most accurate ab initio calculations indicate that the 2A1 van der Waals state is slightly more stable. In contrast, argon matrix isolation experiments have clearly identified the Li-(C2H4) complex exists in the 2B2 state. Some have suggested that argon matrix effects shift the equilibrium toward the 2B2 state. We report the low-temperature synthesis and IR characterization of Lin-(C2H4)m (n = 1, m = 1 and 2) complexes in solid parahydrogen which are observed using the C═C stretching vibration of ethylene in the complex. These results show that under cryogenic hydrogen storage conditions the Li-(C2H4) complex is more stable in the 2B2 electronic state and thus constitutes a potential hydrogen storage material with desirable characteristics.