Laser pulses that act on fragile samples often alter them irreversibly, motivating single-pulse data collection. Amorphous solid water (ASW) is a good example. In addition, neither well-defined paths for molecules to travel through ASW nor sufficiently small samples to enable molecular dynamics modeling have been achieved. Combining nanoimprint lithography and photoinitiation overcomes these obstacles. An array of gold nanoparticles absorbs pulsed (10 ns) 532 nm radiation and converts it to heat, and doped ASW films grown at about 100 K are ejected from atop the irradiated nanoparticles into vacuum. The nanoparticles are spaced from one another by sufficient distance that each acts independently. Thus, a temporal profile of ejected material is the sum of about 106 “nanoexperiments,” yielding high single-pulse signal-to-noise ratios. The size of a single nanoparticle and its immediate surroundings is sufficiently small to enable modeling and simulation at the atomistic (molecular) level, which has not been feasible previously. An application to a chemical system is presented in which H/D scrambling is used to infer the presence of protons in films composed of D2O and H2O (each containing a small amount of HDO contaminant) upon which a small amount of NO2 has been deposited. The pulsed laser heating of the nanoparticles promotes NO2/N2O4 hydrolysis to nitric acid, whose protons enhance H/D scrambling dramatically.

Molecular transport and morphological change were examined in films of amorphous solid water (ASW). A buried N2O4 layer absorbs pulsed 266 nm radiation, creating heated fluid. Temperature and pressure gradients facilitate the formation of fissures through which fluid travels to (ultrahigh) vacuum. Film thickness up to 2400 monolayers was examined. In all cases, transport to vacuum could be achieved with a single pulse. Material that entered vacuum was detected using a time-of-flight mass spectrometer that recorded spectra every 10 μs. An ASW layer insulated the N2O4 layer from the high-thermal-conductivity MgO substrate; this was verified experimentally and with heat-transfer calculations. Laser-heated fluid strips water from fissure walls throughout its trip to vacuum. Experiments with alternate H2O and D2O layers reveal efficient isotope scrambling, consistent with water reaching vacuum via this mechanism. It is likely that ejected water undergoes collisions just above the film surface due to the high density of material that reaches the surface via fissures, as evidenced by complex temporal profiles extending past 1 ms. Little material enters vacuum after cessation of the 10 ns pulse because cold ASW near the film surface freezes material that is no longer being heated. A proposed model is in accord with the data.

Geometric phase is an interesting topic that is germane to numerous and varied research areas: molecules, optics, quantum computing, quantum Hall effect, graphene, and so on. It exists only when the system of interest interacts with something it perceives as exterior. An isolated system cannot display geometric phase. This article addresses geometric phase in polyatomic molecules from a gauge field theory perspective. Gauge field theory was introduced in electrodynamics by Fock and examined assiduously by Weyl. It yields the gauge field Aμ, particle–field couplings, and the Aharonov–Bohm phase, while Yang–Mills theory, the cornerstone of the standard model of physics, is a template for non-Abelian gauge symmetries. Electronic structure theory, including nonadiabaticity, is a non-Abelian gauge field theory with matrix-valued covariant derivative. Because the wave function of an isolated molecule must be single-valued, its global U(1) symmetry cannot be gauged, i.e., products of nuclear and electron functions such as χnψn are forbidden from undergoing local phase transformation on R, where R denotes nuclear degrees of freedom. On the other hand, the synchronous transformations (first noted by Mead and Truhlar): ψn → ψneiζ and simultaneously χn → χne−iζ, preserve single-valuedness and enable wave functions in each subspace to undergo phase transformation on R. Thus, each subspace is compatible with a U(1) gauge field theory. The central mathematical object is Berry's adiabatic connection i〈n|∇n〉, which serves as a communication link between the two subsystems. It is shown that additions to the connection according to the gauge principle are, in fact, manifestations of the synchronous (eiζ/e−iζ) nature of the ψn and χn phase transformations. Two important U(1) connections are reviewed: qAμ from electrodynamics and Berry's connection. The gauging of SU(2) and SU(3) is reviewed and then used with molecules. The largest gauge group applicable in the immediate vicinity of a two-state intersection is U(2), which factors to U(1) × SU(2). Gauging SU(2) yields three fields, whereas U(1) is not gauged, as the result cannot be brought into registry with electronic structure theory, and there are other problems as well. A parallel with spontaneous symmetry breaking in electroweak theory is noted. Loss of SU(2) symmetry as the energy gap between adiabats increases yields the inter-related U(1) symmetries of the upper and lower adiabats, with spinor character imprinted in the vicinity of the degeneracy.

Thin films composed of 400–500 monolayers (ML) of either amorphous solid water (ASW) or ASW/CO2 mixtures are grown atop a MgO(100) substrate under ultrahigh vacuum conditions. Samples are irradiated at an infrared frequency of 3424 cm–1, which lies within the broad OH stretch band of condensed water. Ablation is achieved using 10 ns pulses whose energy (<2.7 mJ) is focused to a beam waist of approximately 0.5 mm. By using a time-of-flight mass spectrometer to monitor ablated material, excellent single-shot detection is demonstrated. This capability is essential because, in general, the first infrared pulse can induce irreversible changes throughout the irradiated volume. With ASW/CO2 samples, CO2 is released preferentially. This is not surprising in light of the metastability of the samples. Indeed, repeated irradiation of the same spot can rid the sample of essentially all of the CO2 in as little as a few pulses, whereas only 10–20 ML of H2O are removed per pulse. The influence of the substrate is profound. It cools the sample efficiently because the characteristic time for heat transfer to the substrate is much less than the infrared pulse duration. This creates temperature gradients, thereby quenching processes such as explosive boiling (phase explosion) and the heterogeneous nucleation of cavities that take place at lower depths in significantly thicker samples, i.e., with sufficient inertial confinement. This efficient quenching accounts for the fact that only 10–20 ML of H2O are removed per pulse. The presence of small protonated water cluster ions in the mass spectra is interpreted as evidence for the trivial fragmentation mechanism examined assiduously by Lewis and co-workers. Mixed samples such as ASW/CO2, where species segregation plays a pivotal role, add interesting and potentially useful dimensions to the ablation phenomenon.

High-n Rydberg time-of-flight spectroscopy has been used to study the 193.3 nm photolysis of AsH3. The center-of-mass translational energy distribution for the 1-photon process, AsH3 + hν → AsH2 + H, P(Ec.m.), indicates that AsH2 internal excitation accounts for ∼64% of the available energy [i.e., hν − D0(H2As − H)]. Secondary AsH2 photodissociation also takes place. Analyses of superimposed structure atop the broad P(Ec.m.) distribution suggest that AsH2 is formed with significant a-axis rotation as well as bending excitation. Comparison of the results obtained with AsH3 versus those of the lighter group-V hydrides (NH3, PH3) lends support to the proposed mechanisms. Of the group-V hydrides, AsH3 lies intermediate between the nonrelativistic and relativistic regimes, requiring high-level electronic structure theory.

A mass spectrometric depletion spectrum (17 700–18 300 cm−1) is reported for NO2 in superfluid (0.37 K) helium nanodroplets. Gas phase NO2 is believed to be vibronically chaotic at these energies. Transitions are broadened and blue-shifted relative to their gas phase counterparts. The spectrum is fitted reasonably well by setting all of the widths and shifts equal to 7 cm−1. Modest dispersions (i.e., 90% lie within 2 cm−1 of the central values) are consistent with quantum chaos in NO2. Relaxation is dominated by interactions of NO2 with its non-superfluid helium nearest neighbors.

Geometric phase is an interesting topic that is germane to numerous and varied research areas: molecules, optics, quantum computing, quantum Hall effect, graphene, and so on. It exists only when the system of interest interacts with something it perceives as exterior. An isolated system cannot display geometric phase. This article addresses geometric phase in polyatomic molecules from a gauge field theory perspective. Gauge field theory was introduced in electrodynamics by Fock and examined assiduously by Weyl. It yields the gauge field Aμ, particle–field couplings, and the Aharonov–Bohm phase, while Yang–Mills theory, the cornerstone of the standard model of physics, is a template for non-Abelian gauge symmetries. Electronic structure theory, including nonadiabaticity, is a non-Abelian gauge field theory with matrix-valued covariant derivative. Because the wave function of an isolated molecule must be single-valued, its global U(1) symmetry cannot be gauged, i.e., products of nuclear and electron functions such as χnψn are forbidden from undergoing local phase transformation on R, where R denotes nuclear degrees of freedom. On the other hand, the synchronous transformations (first noted by Mead and Truhlar): ψn → ψneiζ and simultaneously χn → χne−iζ, preserve single-valuedness and enable wave functions in each subspace to undergo phase transformation on R. Thus, each subspace is compatible with a U(1) gauge field theory. The central mathematical object is Berry's adiabatic connection i〈n|∇n〉, which serves as a communication link between the two subsystems. It is shown that additions to the connection according to the gauge principle are, in fact, manifestations of the synchronous (eiζ/e−iζ) nature of the ψn and χn phase transformations. Two important U(1) connections are reviewed: qAμ from electrodynamics and Berry's connection. The gauging of SU(2) and SU(3) is reviewed and then used with molecules. The largest gauge group applicable in the immediate vicinity of a two-state intersection is U(2), which factors to U(1) × SU(2). Gauging SU(2) yields three fields, whereas U(1) is not gauged, as the result cannot be brought into registry with electronic structure theory, and there are other problems as well. A parallel with spontaneous symmetry breaking in electroweak theory is noted. Loss of SU(2) symmetry as the energy gap between adiabats increases yields the inter-related U(1) symmetries of the upper and lower adiabats, with spinor character imprinted in the vicinity of the degeneracy.