A Spontaneously Electrical State of Matter.

ConspectusMolecular deposition on solid surfaces forms crystalline or amorphous/glassy thin solid films. Intermolecular interactions govern the packing and dynamics of these films. The connection between molecular structure and intermolecular interactions is based on understanding electrostatic forces, dispersion forces and hydrogen bonding. Recently, an entire class of dipolar molecular species have demonstrated counterintuitive self-organization such that the dipole moments of individual molecules are oriented in thin films. This leads to the spontaneous generation of polarized molecular films manifesting a polarization charge equivalent to tens to hundreds of volts in strength at the film-vacuum interface, relative to the film-substrate interface. These voltages, and the corresponding electric fields present in such films, result from a collective and spontaneous orientation of molecular dipoles throughout the film during film growth and represent a metastable state of polarized material. The existence of these materials should encourage reconsideration of the importance of solid-state intermolecular electrostatic interactions.This account will detail observations of the spontaneous electric fields in molecular solids, provide insights into the dynamics and structure of molecular materials that the emergence of these electric fields can facilitate, and present a dipole-alignment based mean-field model that reproduces the temperature dependence of the electric field strength. Species as diverse as carbon monoxide, nitrous oxide, freons, simple alcohols, and cis-methyl formate have been demonstrated to spontaneously generate electric fields. We have reported electric fields more than 108 V m-1, have shown how field strength varies with the film deposition temperature, and have reported temperature-dependent Stark shifts observable in both infrared and ultraviolet absorption spectra. The latter has led to the reporting of large Wannier-Mott excitons in wide band gap molecular materials, such as solid carbon monoxide and ammonia.Changes in the field strength with time, at specific temperatures, can be related to the structural dynamics of glassy molecular solids. Measurement of surface potentials is a very sensitive technique by which to observe the rotation and translation of molecular species buried in thin films. This is particularly true for polarized, supercooled molecular glasses, where surface potentials have been used to report on secondary relaxation processes that have hitherto been hidden from measurement.Characterizing spontaneously electric molecular films, and understanding their behavior, requires the inclusion of nonlocal and nonlinear effects. The mean-field model that we present describes the data by connecting the energy of interaction of an average dipole with the mean effective field in the film, where this field is itself a function of the degree of polarization. This feedback loop produces a smooth function with a nonintuitive, discontinuous differential. The condensation of thin molecular films is an important means by which molecular solids are generated in the interstellar medium and represents a key pathway for the generation of organic optically and electrically active materials. It may also be possible to manipulate chemistry with the intense, localized electric fields acting as or acting on catalysts. The repercussions of the spontaneous generation of bound surface charges and the presence of electric fields in molecular solids will be discussed in these contexts.

Infrared photodesorption of CO from astrophysically relevant ices studied with a free-electron laser.

The infrared excitation and photodesorption of carbon monoxide (CO) and water-containing ices have been investigated using the FEL-2 free-electron laser light source at the FELIX laboratory, Radboud University, The Netherlands. CO-water mixed ices grown on a gold-coated copper substrate at 18 K were investigated. No CO photodesorption was observed, within our detection limits, following irradiation with light resonant with the C-O vibration (4.67 µm). CO photodesorption was seen as a result of irradiation with infrared light resonant with water vibrational modes at 2.9 µm and 12 µm. Changes to the structure of the water ice, which modifies the environment of the CO in the mixed ice, were also seen subsequent to irradiation at these wavelengths. No water desorption was observed at any wavelength of irradiation. Photodesorption at both wavelengths is due to a single-photon process. Photodesorption arises due to a combination of fast and slow processes of indirect resonant photodesorption (fast), and photon-induced desorption resulting from energy accumulation in the librational heat bath of the solid water (slow) and metal-substrate-mediated laser-induced thermal desorption (slow). Estimated cross-sections for the slow processes at 2.9 µm and 12 µm were found to be ∼7.5 × 10-18 cm2 and ∼4.5 × 10-19 cm2, respectively.

A theoretical study on spontaneous dipole orientation in ice structures.

Spontaneous dipole orientation is studied for a set of simulated porous ASW ice films on a substrate held at temperatures ranging from 10 K to 140 K. It is found that the water dipoles in the films obtained at the lower temperatures are oriented such that a negative electric field with a magnitude of 108-109 V m-1 is obtained. The magnitude of the field increases approximately linearly with height above the substrate, akin to experimental observations, although the magnitude of our field increases faster. A strong temperature dependence of the surface potential resulting from the spontelectric field is found, where the surface potential decreases when the substrate temperature increases. The surface potential finally becomes close to zero for temperatures around and above 110 K.

A new technique for determining the refractive index of ices at cryogenic temperatures.

A reflection-absorption optical (RAO) spectrometer, operating across the ultra-violet/visible (UV/visible) wavelength region, has been developed that allows simultaneous measurements of optical properties and thickness of thin solid films at cryogenic temperatures in ultrahigh vacuum. The RAO spectrometer enables such measurements to be made after ice deposition, as opposed to most current approaches which make measurements during deposition. This allows changes in the optical properties and in the thickness of the film to be determined subsequent to thermal, photon or charged particle processing. This is not possible with current techniques. A data analysis method is presented that allows the wavelength dependent n and k values for ices to be extracted from the reflection-absorption spectra. The validity of this analysis method is shown using model data from the literature. New data are presented for the reflection UV/visible spectra of amorphous and crystalline single component ices of benzene, methyl formate and water adsorbed on a graphite surface. These data show that, for benzene and methyl formate, the crystalline ice has a larger refractive index than amorphous ice, reflecting changes in the electronic environment occurring in the ice during crystallisation. For water, the refractive index does not vary with ice phase.

Correction: Surface heterogeneity and inhomogeneous broadening of vibrational line profiles.

Correction for 'Surface heterogeneity and inhomogeneous broadening of vibrational line profiles' by Skandar Taj et al., Phys. Chem. Chem. Phys., 2017, 19, 7990-7995.

Surface heterogeneity and inhomogeneous broadening of vibrational line profiles.

The surface heterogeneity of amorphous silica (aSiO2) has been probed using coverage dependent temperature programmed desorption (TPD) of a simple probe molecule, carbon monoxide (CO). The resulting distribution of interaction energies is the foundation from which an environmentally broadened vibrational line profile synthesis has been undertaken. These simulations are compared with measured line profiles recorded at 0.1 cm-1 resolution using reflection-absorption infrared spectroscopy (RAIRS). A comparison of such line profile synthesis for CO on amorphous silica and on porous amorphous solid water (p-ASW) is also reported and conclusions are drawn as to the vibrational relaxation and surface dynamics of the CO molecule on the two surfaces.

Molecular hydrogen production from amorphous solid water during low energy electron irradiation.

This work investigates the production of molecular hydrogen isotopologues (H2, HD, and D2) during low energy electron irradiation of layered and isotopically labelled thin films of amorphous solid water (ASW) in ultrahigh vacuum. Experimentally, the production of these molecules with both irradiation time and incident electron energy in the range 400 to 500 eV is reported as a function of the depth of a buried D2O layer in an H2O film. H2 is produced consistently in all measurements, reflecting the H2O component of the film, though it does exhibit a modest reduction in intensity at the time corresponding to product escape from the buried D2O layer. In contrast, HD and D2 production exhibit peaks at times corresponding to product escape from the buried D2O layer in the composite film. These features broaden the deeper the HD or D2 is formed due to diffusion. A simple random-walk model is presented that can qualitatively explain the appearance profile of these peaks as a function of the incident electron penetration.

Non-covalent interaction of benzene with methanol and diethyl ether solid surfaces.

We present laboratory experiments on binary, layered ices comprised of benzene (C6H6) on methanol (CH3OH) and on diethyl ether (CH3CH2OCH2CH3). Temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) have been used to investigate the growth mechanisms in these systems. Ab initio quantum chemical calculations on simple gas-phase model clusters are used to aid interpretation of the experimental data by highlighting the key interactions established at the interface. Our observations are consistent with C6H6 forming islands on CH3OH, although evidence of strong hydrogen bonding interactions indicates some degree of surface wetting. In contrast, layer-by-layer growth is proposed for C6H6 on the CH3CH2OCH2CH3 substrate.

Electrons, excitons and hydrogen bonding: electron-promoted desorption from molecular ice surfaces.

Desorption of benzene (C6H6) from thick methanol (CH3OH) and diethyl ether (CH3CH2OCH2CH3) ices during irradiation with 250 eV electrons is reported and compared with our previous work on C6H6 desorption from water (H2O) ice systems. C6H6 electron-promoted desorption (EPD) is seen to be sensitive to the chemical nature of the substrate reflecting both the importance of the excitations localised around the O-atom versus those involving the C-atom; and the role of hydrogen bonding interactions in transporting non-dissociative electronic excitation to the substrate/C6H6 interfaces during the electron irradiation.

Efficient electron-promoted desorption of benzene from water ice surfaces.

Desorption of benzene (C6H6) from solid water surfaces [compact amorphous solid water (c-ASW) and crystalline ice (CI)] during irradiation of ultrathin solid films with low energy (250-300 eV) electrons has been investigated. The observed desorption behaviour is complex but typically two desorption components, with particularly large cross-sections, were present in the observed signal. A fast component, with a cross-section up to 10(-15) cm(2), is attributed to desorption of isolated C6H6 molecules that are hydrogen-bonded to small clusters of water (H2O) molecules on the solid water surface. A slower component, with a cross-section of ca. 10(-17) cm(2), is attributed mainly to desorption from larger C6H6 islands on the solid water surface. Possible desorption mechanisms are proposed and astrophysical implications are discussed.

Spontaneous polarization of solid CO on water ices and some astrophysical implications.

Reflection absorption infrared spectroscopy (RAIRS) is used to show that when 20 monolayer (ML) films of solid CO are laid down on solid water substrates at 20 to 24 K, the films polarize spontaneously. CO films were prepared on three types of water ice: porous amorphous solid water (CO-pASW), crystalline water (CO-CSW) and compact amorphous solid water (CO-cASW) with corresponding fields of 3.76 ± 0.15 × 10(7) V m(-1) for CO-pASW, 2.87 ± 0.15 × 10(7) V m(-1) for CO-CSW and 1.98 ± 0.15 × 10(7) V m(-1) for CO-cASW. For comparison, CO laid down on SiO2 yields 3.8 ± 0.15 × 10(7) V m(-1). Our results are of relevance to an understanding of the chemistry and physics of dense star-forming regions in the interstellar medium, in which dust particles become coated with solid CO on a layer of cASW. The polarization charge which accumulates on the CO surface acts as a catalyst for the removal of electrons and ions from the medium and may account for the low degree of ionization observed in these regions, a feature which is an important factor for the rate of star formation.

Peeling the astronomical onion.

Water ice is the most abundant solid in the Universe. Understanding the formation, structure and multiplicity of physicochemical roles for water ice in the cold, dense interstellar environments in which it is predominantly observed is a crucial quest for astrochemistry as these are regions active in star and planet formation. Intuitively, we would expect the mobility of water molecules deposited or synthesised on dust grain surfaces at temperatures below 50 K to be very limited. This work delves into the thermally-activated mobility of H2O molecules on model interstellar grain surfaces. The energy required to initiate this process is studied by reflection-absorption infrared spectroscopy of small quantities of water on amorphous silica and highly oriented pyrolytic graphite surfaces as the surface is annealed. Strongly non-Arrhenius behaviour is observed with an activation energy of 2 kJ mol-1 on the silica surface below 25 K and 0 kJ mol-1 on both surfaces between 25 and 100 K. The astrophysical implication of these results is that on timescales shorter than that estimated for the formation of a complete monolayer of water ice on a grain, aggregation of water ice will result in a non-uniform coating of water, hence leaving bare grain surface exposed. Other molecules can thus be formed or adsorbed on this bare surface.

Spontaneous electric fields in solid carbon monoxide.

Reflection-absorption infrared spectroscopy (RAIRS) is shown to provide a means of observing the spontelectric phase of matter, the defining characteristic of which is the occurrence of a spontaneous and powerful static electric field within a film of material. The presence of such a field is demonstrated here through the study of longitudinal-transverse optical splitting in RAIR spectra in films of carbon monoxide, based upon the deposition temperature dependence of this splitting. Analysis of spectral data, in terms of the vibrational Stark effect, allows the measurement of the polarization of spontelectric films, showing for example that solid carbon monoxide at 20 K may maintain a spontelectric field of 3.78 × 10(7) V m(-1), representing a polarization of 3.34 × 10(-4) cm(-2). We comment on the astrophysical implications of polarized carbon monoxide ices, on the surface of cosmic grains in star-forming regions.

Spontaneously electrical solids in a new light.

Reflection-absorption infrared spectroscopy (RAIRS) of nitrous oxide (N2O) thin films is shown to provide an independent means of observing the spontelectric state, the first new structural phase of matter, with unique electrical properties, to have emerged in decades. The presence of a spontaneous and powerful static electric field within the film, the defining characteristic of spontelectric solids, is demonstrated through observations of longitudinal-transverse optical (LO-TO) splitting in RAIR spectra, using an analysis based on the vibrational Stark effect. In particular the dependence of the LO-TO splitting on the film deposition temperature may be wholly attributed to the known temperature dependence of the spontelectric field.

The interaction of carbon monoxide with model astrophysical surfaces.

Carbon monoxide (CO) is an important component of the icy mantles that accrete on interstellar dust grains. To develop a better understanding of the physicochemical basis of its infrared spectroscopy, we have studied the interaction of submonolayer coverages of CO with the surface of films of other astrophysically relevant species--(13)CO, carbon dioxide (CO2), ammonia (NH3), methanol (CH3OH) and water (H2O)--under ultrahigh vacuum and cryogenic (10 K) conditions using reflection-absorption infrared spectroscopy (RAIRS). In support of these measurements, we have performed ab initio calculations of gas phase dimer complexes, and made comparisons to experimental results of gas phase and matrix isolated complexes, which are extensively reported in the literature. The interaction of CO can be categorised as occurring via the C atom (C(CO) bonded), the O atom (O(CO) bonded) or in a π-bonded configuration. The C(CO) configuration is characterised by a blue shifted C≡O stretch frequency, and is observed for CO adsorbed on (13)CO, CO2 and H2O surfaces. From the absence of such a feature from the spectra of CO adsorbed on CH3OH it can be concluded that the dangling OH bonds required for this adsorption configuration are not present at the surface of the CH3OH film.