TOF spectra of H2O desorbed from ASW by 157-nm photons with three different desorption laser spot sizes. The desorption laser beam passed through an iris opening with a radius of 1 mm (black circles), 2 mm (green squares), and 3 mm (blue triangles). The fits in (a) were calculated using Eq. (4) with r = 2 mm, T trans = 625 ± 5 K, and R max ∼ 4 mm (exact values in legend). In (b), the TOF spectra were scaled and overlaid to emphasize their indistinguishable shapes. The inset shows a nearly linear relationship between integrated water signal (from 0 to 10 μs) and desorption laser spot area.
Experimental 2 + 1 REMPI spectrum of H2O desorbed from ASW by 157-nm photons at 108 K, measured at r = 3.1 mm and t = 2.8 μs (dots), and simulated two-photon C-X spectrum with a rotational temperature of 425 K and an ortho:para ratio of 3:1 (line). The rovibrational transitions associated with the five largest peaks are shown.
TOF spectrum of H2O desorbed from ASW at 108 K by 157-nm photons and detected by 2 + 1 REMPI on the 202-321 rotational peak at 248.52 nm. The solid line is a Maxwell-Boltzmann distribution with r = 3.1 mm and T trans = 700 K.
Velocity distribution of H2O desorbed from ASW at 108 K by 157-nm photons and detected by 2 + 1 REMPI on the 202-321 rotational peak at 248.52 nm. Three different distances between sample and detection region yielded consistent results.
Water signal detected by 2 + 1 REMPI on the 202-321 rotational peak integrated from 2.3 to 3.9 μs delay time as a function of energy per pulse. The two sets of data were obtained with slightly different ionization energies. The solid lines are linear regressions.
Representative cross section measurements for H2O removal from 5 and 20 L ASW by 157-nm irradiation. A 5 L data and exponential fitting curve were offset for clarity. The number of 157-nm photons was calculated using the pulse counter on the excimer laser display and an average energy of 55 μJ cm−2 per pulse.
Average cross sections for H2O removal from 1 to 30 L ASW by 157-nm irradiation. Error bars represent 95% confidence intervals based on 4–6 different measurements at each exposure.
TOF spectra of (a) OH+ and (b) H+ fragment ions from vibrationally excited H2O molecules desorbed by 157-nm photons, ionized and fragmented by 253-nm photons. A Maxwell-Boltzmann distribution with T trans = 1400 K fits the OH+ data, and a distribution with T trans = 1200 K fits the peak of the H+ spectrum.
Electron attachment/detachment densities of the third excited singlet state of the model tetramer and the first excited state of the tetramer in the QM/MM model system (CIS/aug-cc-pVDZ). Blue represents electron attachment density, while red represents a region of depleted electron density.
Ground-state (S0) and excited state (S1-S4) potential energy curves of rigid OH stretching of a surface water molecule in the QM/MM model of hexagonal ice, calculated using EOM-CCSD/aug-cc-pVDZ for the quantum region and the TIP3P for the surrounding water molecules.
Force and energy (inset) plots constructed for the S1 potential of the QM/MM model at the CIS/aug-cc-pVDZ level of theory. The central water of the quantum mechanical tetramer was pulled rigidly away from the surface. The force plot was used to estimate the maximum translational energy of a desorbing water molecule.
Translational energy distribution of H2O desorbed from ASW at 108 K by 157-nm photons and detected by 2 + 1 REMPI at three different distances between the sample and the detection region. Upper bounds estimated at the CIS/aug-cc-pVDZ level of theory are shown at A (0.144 eV) and B (0.202 eV).
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