Detecting Hydrogen Atoms in Solid Parahydrogen using FTIR Spectroscopy RD03 - Cold Quantum Systems 1015 McPherson Lab 9:22 am Thursday, June 21, th International Symposium on Molecular Spectroscopy This research sponsored in part by the Chemistry Division of the National Science Foundation (CHE ). David T. Anderson and Mahmut Ruzi Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA. pH 2

Chemistry happens in the high energy tail of the Maxwell-Boltzmann distribution E a = 2130 cm -1 = +6.1 kcal/mol  H 0 = cm -1 = kcal/mol 700 K 100 K OH + H 2 → H 2 O + H

H 2 O···H What happens at really low temperatures? intermolecular wells OH + H 2 → H 2 O + H k H-atom tunneling H-atom tunneling MD Wheeler, DT Anderson and MI Lester Int. Rev. Phys. Chem. 19, 501 (2000). The entrance channel complex OH···H 2 is stable for 100s of  sec!

Study low temperature reactions in solid parahydrogen T = 1.9 K pH 2 H-atom 3.8 Å OH + H 2 → H 2 O + H H2OH2O OH Can we use the IR spectrum of H 2 O to detect the presence of the H-atom? or the vacancy left behind?

Detecting H-atoms in solid H 2 with ESR Spectroscopy T. Kumada, Phys. Rev. B 68, (2003) mT H 3 ppm O 2 -doped solid H 2 UV = Hg lamp T = 3.4 K Rate=k[H] 2

H-atom moves in solid H 2 via chemical tunneling H + pH 2 → pH 2 + H  Tun   H + H 2 → H 2 + H E a = 10 kcal/mol T. Kumada, Phys. Rev. B 68, (2003). Somewhat analogous to the grotthuss mechanism for proton motion in H 2 O

Photochemical experimental setup atmosphere vacuum FTIR beam radiation shield optical substrate pH 2 crystal pH 2 gas dopant gas UV beam Liquid helium cryostat 1.7 to 4.5 K

Photochemistry of the simplest carboxylic acid HCOOH + h → CO + H 2 O 6.3 → CO 2 + H → HCO + OH 96.8 → HCOO + H → H + COOH 91.1 S 1 ←S 0  193nm)=0.8x cm 2 H. Su et al., J. Chem. Phys. 113, 1891 (2000). Formic acid (FA) (trans)  H°(kcal/mol) E photon  193nm)=148 kcal/mol CO(n→  *)

IR spectra in the 3 H 2 O region *K. Kufeld, W. Wonderly, L. Paulson, S. Kettwich, and DTA, JPCL 3, 342 (2012) before after 7 hours later Detect H 2 O R(0) satellite peaks after photolysis that decay with time!

Experiments with fully deuterated formic acid – effect on H 2 O satellite peaks DCOOD + h → CO + D 2 O → CO 2 + D 2 → DCO + OD → DCOO + D → D + COOD 91% <1% → HDO + H +H 2 reactions with pH 2 host percentages based on HCOOH photolysis experiments

HDO, D 2 O, and H 2 O satellite peaks No satellite peaks detected for D 2 O photoproduct!

IR spectroscopy of gas-phase Ar···H 2 O vdW complex* 0 00 v 3 =0 v 3 = R(0) pH 2 O Ar-pH 2 O  (0 00 )  (1 01 )  (1 01 ) Ar J=0 M=0 Ar J=1 M=0 Ar J=1 M=±1 D0D0 *O. Votava, SR Mackenzie, DJ Nesbitt, JCP 120, 8443 (2004). J KaKc

IR spectroscopy of Ar···H 2 O similar to satellite peaks R(0) pH 2 O v 3 =0 v 3 =1 S2 S1 R(0) S1 S2 proximity to R(0) peak same decay time constant relative intensities stable = common ground state level

HDO Satellite peaks consistent with H 2 O 1 HDO 3 HDO HDO satellite peaks decay with same time constant as H 2 O satellite peaks HDO R(0) peak grows with same time constant as well Spectroscopic stability between HDO satellite and R(0) transition intensities HDO satellite → HDO R(0)

What we think is going on … DCOOD + h → CO + D 2 O → DCO + OD → HDO + H +H 2 abstraction reaction with pH 2 host → OH + HD → H 2 O + H +H 2 exchange reaction +H 2 → DCO + OD no satellite peaks!

FA/pH nm HCO/OH/pH 2 escape cage fragments thermalize 100s of ns 2 tunneling rxn with pH 2 host 100  s – 100 ms HCO/H∙∙∙H 2 O/pH 2 FA pH 2 OH HCO H 3 H-atom gets displaced H-atoms diffuse 10 mins – 10 hours H/H 2 O/HCO/pH 2 Carriers of the satellite peaks!

Photochemistry in solid pH 2 - timescales chemical energy reaction coordinate H-atom tunneling V0V0 OH pH 2 H2OH2O H right after photolysis 100s of nsec H-atom tunneling rxn produces H 2 O···H exit channel complex dissipation H-atom diffuses away  sec minutes

Conclusions and future directions HDO R(0) satellite peaks are produced in the 193 nm in situ photolysis of d2-formic acid in solid pH 2 at 1.9 K Detailed assignment of the satellite peaks should provide important clues about the photochemical mechanism Detect satellite peaks also in the 193 nm in situ photochemistry of NH 3 (more in RD05) Observe subsequent H-atom chemistry – what role do the carriers of the satellite peaks play (if any)?

Kylie A. Kufeld visiting undergrad Dartmouth College Mahmut Ruzi aka - Mailhemuti UW Graduate Student The people who do the work and funding This work was started by Sharon C. Kettwich (PhD 2010) and Dr. Leif O. Paulson (PhD 2011). This research is sponsored in part by the Chemistry Division of the US National Science Foundation (CHE ). William R. Wonderly 2011 REU student U. of Puget Sound

pH 2 Challenge to theory – Can these two potential satellite peak carriers be spectroscopically distinguished? H 2 O···H H 2 O next to an H-atom H 2 O··· H 2 O next to a vacancy

NH 3 satellite peaks also observed

Fast acquisition FTIR scans right after photolysis 3 NH 3 NH 2 + pH 2 → NH 3 + H NH 2 S1S1 S2S2 R(0) during photolysis 25 sec after 102 sec after Watch NH 2 peak decay and satellite peaks increase!

NH 3 /pH 2 NH 3 pH 2 H escape cage fragments thermalize 100s of ns nm NH 2 /H/pH 2 2 tunneling rxn with pH 2 host ~50 s H/H∙∙∙NH 3 /pH 2 3 H-atom gets displaced H-atoms diffuse 10 mins – 10 hours 2H/NH 3 /pH 2 NH 2 Carriers of the satellite peaks!