Rare Gas Matrices Research Articles

Spectroscopy of the copper dimer in normal fluid, superfluid, and solid H4e

Copper atoms and molecules are laser ablated into bulk liquid and solid helium, and the emission spectra of the laser excited D→X, B→X, and a→X transitions of Cu(2) are observed to exhibit clearly resolved vibrational bands. Surprisingly, for the D→X and the B→X transitions, no differences were observed for superfluid He at 1.5 K, for the normal liquid at 2.65 K, or for the 1.5 K solid at higher pressures of about 30 bars. An interpretation based on the bubble model indicates that the interaction with the He matrix is much weaker than in the case of the alkali atoms. Compared to other solid rare gas matrices, the line shifts and line widths in condensed helium are much smaller by nearly an order of magnitude.

Formic and acetic acids in a nitrogen matrix: Enhanced stability of the higher-energy conformer

Formic acid (HCOOH, FA) and acetic acid (CH(3)COOH, AA) are studied in a nitrogen matrix. The infrared (IR) spectra of cis and trans conformers of these carboxylic acids (and also of the HCOOD isotopologue of FA) are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the νOH and τCOH modes and the relative intensities of the νOH and νC=O bands.

ChemInform Abstract: Generation and Study of Inorganic Cations in Rare Gas Matrixes by ESR

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Electron impact dissociation of N 2O and CO 2 with single particle detection of O( 1D2)

Production of metastable O( 1 D 2) atoms following controlled electron impact on N 2O and CO 2 targets has been studied using a neon rare gas matrix detector operating at a temperature of <20 K. A 100 eV pulsed electron beam was used in conjunction with time-of-flight (TOF) techniques to establish O-atom fragment kinetic energies. Probable dissociation channels are discussed.

Electronic absorption and phosphorescence of cyanodiacetylene

Electronic absorption and emission spectra have been investigated for cyanodiacetylene, HC(5)N, an astrophysically relevant molecule. The analysis of gas-phase absorption was assisted with the parallel rare gas matrix isolation experiments and with density functional theory (DFT) predictions concerning the excited electronic states. Mid-UV systems B (1)Delta<--X (1)Sigma(+) (origin at 282.5 nm) and A (1)Sigma(-)<--X (1)Sigma(+) (306.8 nm) were observed. Vibronic assignments have been facilitated by the discovery of the visible phosphorescence a (3)Sigma(+)<--X (1)Sigma(+) in solid Ar, Kr, and Xe. Phosphorescence excitation spectra, as well as UV absorption measurements in rare gas matrices, revealed the enhancement of A<--X transitions. The vibronic structure of dispersed phosphorescence spectra supplied new data concerning the ground state bending fundamentals of matrix-isolated HC(5)N. The experimental singlet-triplet splitting, 2.92 eV in Ar, closely matches the value of 3.0 eV predicted by DFT.

Unveiled optical properties of tetrapyrollic pigments in cryogenic environments

An unexpected phenomenon was revealed in the laser induced fluorescence spectra of free-base (H2Pc) and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices under a moderate increase in the laser intensity. In all matrices the intensity of an emission band near 755nm increased drastically when pumping the S1←S0 transition. This observation was assigned to stimulated emission in a four-level scheme involving a vibronic transition from the lowest electronic state to a vibrational level of the ground state. In the present work, we expose new similar results obtained with porphyrin molecules, i.e. tetra-benzoporphin (TBP). With free-base H2TBP, stimulated emission was observed in Ar or N2 matrices, but not in Xe matrices. A possible reason could be a fast inter-system crossing rate due to the heavy atom effect induced by Xe. We also report the observation of persistent burnt spectral holes, although the low efficiency of this process is not competitive with stimulated emission and no decrease of the stimulated emission with time was observed. With ZnTBP no stimulated emission could be recorded but the appearance of rather strong phosphorescence was noticed. Similar studies focusing on the stimulated emission phenomenon is that they should allow new insights into site effects and site selectivity.

Metastable oxygen atom detection using rare gas matrices

The use of solid rare gas matrices as detectors of metastable oxygen atoms is investigated. A 100 eV electron beam colliding with N2O target gas is used as the source of O(1S). The parameters considered are surface temperature, time delay of excimer emission and spectral response to O(1S). In all cases, detector sensitivity maximized at temperatures ⩽20 K. Krypton was found to provide the most sensitive surface and neon the least.

Small Magnesium Clusters: Between van der Waals and Valence Bonds

The magnesium molecules Mg(2), Mg(3), Mg(4), and Mg(5) have been isolated in rare-gas matrixes at 8 K and characterized for the first time by Raman spectroscopy. The vibrational frequencies were compared with those obtained by density functional theory calculations. The fundamental frequency of the dimer was observed at 97 cm(-1) (Ar matrix), and a bond length of d(Mg-Mg) = 340.8 pm was calculated. The Mg(3) cluster possesses a triangular structure (D(3h)) with calculated d(Mg-Mg) = 317.8 pm. For tetramagnesium, a tetrahedral structure (T(d)) with d(Mg-Mg) = 301.3 pm is predicted. The Mg(5) molecule possesses a trigonal-bipyramidal structure (D(3h)) with d(Mg-Mg) = 293.1 and 322.5 pm, respectively. The red tetramagnesium cluster is an example of a matrix-isolated magic number cluster.

Periodic bond breaking and making in the electronic ground state on a sub-picosecond timescale: OH bending spectroscopy of malonaldehyde in the frequency domain at low temperature

Proton tunneling between the two equivalent structures of malonaldehyde through a substantial barrier is accelerated by more than a factor of 3 to approximately 0.24 ps by OH-bend excitation in phase with suitable motions of the molecular backbone. This is derived from a combined FTIR and Raman spectroscopy study in supersonic jets and rare gas matrices and compared to previous theoretical predictions.

Charge state of metal atoms on oxide supports: a systematic study based on simulated infrared spectroscopy and density functional theory

A long standing question in the study of supported clusters of metal atoms in the properties of metal–oxide interfaces is the extent of metal–oxide charge transfer. However, the determination of this charge transfer is far from straight forward and a combination of different methods (both experimental and theoretical) is required. In this paper, we systematically study the charging of some adsorbed transition metal atoms on two widely used metal oxides surfaces [α-Al2O3 (0001) and rutile TiO2 (110)]. Two procedures are combined to this end: the computed vibrational shift of the CO molecule, that is used as a probe, and the calculation of the atoms charges from a Bader analysis of the electron density of the systems under study. At difference from previous studies that directly compared the vibrational vawenumber of adsorbed CO with that of the gas phase molecule, we have validated the procedure by comparison of the computed CO stretching wavenumbers in isolated monocarbonyls (MCO) and their singly charged ions with experimental data for these species in rare gas matrices. It is found that the computational results correctly reproduce the experimental trend for the observed shift on the CO stretching mode but that care must be taken for negatively charged complexes as in this case there is a significative difference between the total charge of the MCO complex and the charge of the M atom. For the supported adatoms, our results show that while Cu and Ag atoms show a partial charge transfer to the Al2O3 surface, this is not the case for Au adatoms, that are basically neutral on the most stable adsorption site. Pd and Pt adatoms also show a significative amount of charge transfer to this surface. On the TiO2 surface our results allow an interpretation of previous contradictory data by showing that the adsorption of the probe molecule may repolarize the Au adatoms, that are basically neutral when isolated, and show the presence of highly charged Auδ+–CO complexes. The other two coinage metal atoms are found to significatively reduce the TiO2 surface. The combined use of the shift on the vibrational frequency of the CO molecule and the computation of the Bader charges shows to be an useful tool for the study the charge state of adsorbed transition metal atoms and allow to rationalize the information coming from complementary tools.

Direct absorption spectroscopy of water clusters formed in a continuous slit nozzle expansion

In this article, we report on a Fourier transform infrared study of absorption bands belonging to small-sized water clusters formed in a continuous slit nozzle expansion of water vapor seeded in argon carrier gas. Clear signatures of free and H-bonded OH vibrations in water aggregates from dimer to pentamer are seen in our spectra. Following an increase in argon backing pressure, the position of the cluster absorption bands varies from those characteristics of isolated water aggregates in the gas phase to those known for clusters trapped in a static argon matrix. These variations can be interpreted in terms of sequential solvation of the water clusters by an increasing number of argon atoms attached to water clusters. Our measured spectra are in good agreement with those obtained previously either for free or Ar coated small-sized water clusters using pulsed slit-jet expansions. Our results are equally in accord with those originating from a variety of tunable laser based techniques using molecular beams or free jets or from the study of water aggregates embedded in rare gas matrices. Distinctions are reported, however, and discussed. Ab initio calculations have made it possible to speculate on the average size of an argon solvation shell around individual clusters as well as on the development of the OH stretch vibrational shifts in mixed (H(2)O)(m)Ar(n) clusters having different compositions and architectures.

UV/visible spectroscopy of matrix-isolated hexa-peri-hexabenzocoronene: Interacting electronic states and astrophysical context

Absorption spectra of hexa-peri-hexabenzocoronene isolated in rare-gas matrices are reported for the wavelength range between 200 and 500 nm. Measurements were carried out in neon and in argon at 5.8 and 12.0 K, respectively. Calculations based on semiempirical models and on density-functional theory were performed to assign the observed features. The electronically excited states involved in Clar's alpha- and p-bands are identified as S(1)(B(2u)) and S(2)(B(1u)), respectively. Although the upper state associated with the beta-band is found to be a (1)E(1u) state, it remains undetermined whether it is S(3) or S(4). Structures in the beta-band are interpreted as resulting from the interaction between the (1)E(1u) state and the e(2g) vibrational manifold of S(2)(B(1u)). The new measurements are used to narrow down the wavelength ranges where the bands of hexa-peri-hexabenzocoronene should be found in the gas phase. A previous estimate of the interstellar abundance of this polycyclic aromatic hydrocarbon is discussed.

Femtosecond response in rare gas matrices doped with NO impurities: A stochastic approach

The femtosecond response of NO-doped rare gas matrices is studied within a stochastic Langevin theoretical framework. As is shown, a simple damped harmonic oscillator model can describe properly the absorption and emission line shapes associated with the NO ( A 2 Σ + ↔ X 2 Π ) electronic transitions inside these media as well as the matrix first solvation shell response in a process with two timescales, finding a fairly good agreement with available experimental data. This approach thus constitutes an alternative and complementary way to analyze the structural relaxation dynamics of systems in liquids and solids, leading to a better understanding of the underlying physics.

Rare gas matrix detector for molecular fragmentation studies

Fragment-specific detection following electron or photon dissociation of molecules is often an essential element for accurate characterization of the dissociation pathways and energetics. Previously, a Xe matrix has proved to be a sensitive and selective detector for oxygen or sulphur 1S0 metastable atoms. In the present work the use and performance of other rare gas matrices for metastable particle detection is evaluated and quantified.

Distorted forms of metalloporphyrins probed by methods of highly resolved spectroscopy

The spectral manifestations of the distorted forms of sterically unconstrained metallocomplexes of porphyrin in rare gas and Shpol'skii matrices at cryogenic temperatures were detected. The planar and two kinds of distorted conformations of the Mg - and Zn -porphyrins in rare gas matrices have been measured simultaneously in the fluorescence and phosphorescence spectra due to external heavy atom effect. The distinctly different frequencies of the vibronic transitions have been revealed for planar and distorted forms. In the phosphorescence spectra of the Pd -porphyrin the manifestation of the two forms in the ground state has been proven also, with one of them being planar and the other being saddle-type distorted. The ratio of the planar and non-planar forms was shown to depend on the deuteration of both meso-positions of the porphyrin and n-alkane matrix. The appearance of the out-of-plane modes in the phosphorescence spectra of the Pd - and Pt -porphyrin has been demonstrated.

Ultrafast nonadiabatic photodissociation dynamics of F2 in solid Ar

We explore the ultrafast spin-flip dynamics in a diatomic molecule imbedded in a rare gas matrix using the combination of a quantum mechanical and a semiclassical surface hopping method. Specifically, we investigate (1) the extent to which the phenomenon of electronically-localized eigenstates in strongly-coupled manifolds survives in the presence of rapid decay and a multitude of electronically coupled states; (2) the ability of the surface hopping method to predict the short time dynamics; and (3) the time range over which frozen lattice models are valid. Our results point to the active role played by a large number of coupled electronic states in the F2/Ar dynamics while substantiating our confidence in the validity of the popular surface hopping approach for the system considered.

Dynamics of exciplex formation in rare gas media

A hopping-surface algorithm has been used to simulate the dynamics induced in rare gas matrices due to the photoexcitation ( 1S 0 → 3P 1) of atomic mercury embedded in them. Especially, the study of the dynamics of an exciplex formation in a model system consisting of solid xenon doped with atomic mercury. The process starts upon the photoexcitation of the Hg atom to its 3P 1 electronic excited state. Diatomics-in-Molecule approach has been used for constructing the adiabatic potential surfaces. In all trajectories we show that a triatomic Xe–Hg ∗–Xe complex is formed, but in two conformations: bent and linear. The mechanisms leading to the formation of one or the other are identified. Mainly, are noted the thermal fluctuations of the Hg impurity and the shape of the potential surfaces. Furthermore, we show that non-radiative intrastate relaxation occurs via a conical intersection between the excited state surfaces. The simulated spectra are in very good agreement with the experimental data.

Formic Acid Dimerization: Evidence for Species Diversity from First Principles Simulations

Dimerization of formic acid has been simulated using ab initio molecular dynamics at conditions mimicking rare gas matrix isolation experiments. Aggregation product distributions and the corresponding reaction pathways have been studied as a function of temperature. At higher temperatures, the cyclic, C(2h) symmetric, global minimum structure A with two O-H...O horizontal lineC hydrogen bonds predominates over the second most stable, acyclic, local minimum isomer B with one O-H...O horizontal lineC and one C-H...O horizontal lineC hydrogen bond, whereas the latter is the main species at low temperature. Significant concentrations of two additional, less stable, local minimum species, C exhibiting an O-H...O horizontal lineC and an O-H...O(H)-C hydrogen bond, and D with an O-H...O(H)-C and a C-H...O horizontal lineC hydrogen bond, which should be detectable in experiment, are predicted at low temperature. Theoretical vibrational spectra are provided to guide the experimental search for these species. Furthermore, free-energy barriers for thermal interconversion of different dimer species have been calculated using targeted molecular dynamics in conjunction with thermodynamic integration. The small barrier for the C --> A reaction of 7.0 kJ/mol indicates that the C species can only be stabilized at ultracold conditions. The data presented here thus hold important clues for the execution and interpretation of low-temperature vibrational spectroscopy at matrix isolation or ultracold helium droplet conditions.

Spectroscopic study of cis-to-trans tunneling reaction of HCOOD in rare gas matrices

The higher energy conformer (cis) of HCOOD is prepared by vibrational excitation of the trans form. The cis conformer decays back to the conformational ground state (trans) via tunneling of deuterium. The tunneling process in HCOOD in rare gas matrices is extremely slow (in scale of weeks). We present new measurements of the tunneling rate constants, which characterize the efficiency of the cis-to-trans conversion process in Ne, Ar, Kr, and Xe matrices. The tunneling rates of HCOOD follow the trend k(Xe) approximately = k(Kr)>k(Ar) approximately = k(Ne), which is anomalous with respect to the reaction barrier of the solvated molecule. We propose a semiempirical energetic scheme of solid state solvation, which is consistent with all experimental observation. The temperature dependence of the tunneling constants rates of HCOOD is very weak compared to HCOOH in all matrices. The fundamental vibrational frequencies of the cis and trans conformers of HCOOD in various matrices are reported.

Vibrational Relaxation of Matrix-Isolated Carboxylic Acid Dimers and Monomers

Femtosecond mid-IR transient absorption spectroscopy was used to probe the vibrational dynamics of formic acid and acetic acid isolated in solid argon following excitation of the fundamental transition of the carbonyl stretching mode. Carboxylic acids form extremely stable H-bonded dimers, hindering the study of the monomeric species at equilibrium conditions. The low-temperature rare-gas matrix isolation technique allows for a unique control over aggregation enabling the study of the monomer vibrational dynamics, as well as the dynamics of two distinct dimer structures (cyclic and open chain). This study provides insight into the role of the methyl rotor and hydrogen bonding in the vibrational dynamics of carboxylic acids. In the monomer of FA, depopulation of the initially excited state is characterized by a time constant of approximately 500 ps, and it is followed by the energy transfer from intermediately populated intramolecular vibrational states into the phonon modes of the argon lattice (vibrational cooling) in a much longer time scale (estimated to be longer than 5 ns). The methyl rotor in acetic acid monomer accelerates both processes of population transfer and vibrational cooling, with time constants of approximately 80 ps. Hydrogen bonding in formic acid dimers decreases the time constant associated with the dominant vibrational relaxation process by more than 2 orders of magnitude. Unlike in formic acid, hydrogen bonding in acetic acid has no apparent effect on the vibrational cooling rate.