Molecular Beam Expansion Research Articles

Chiral clusters in a supersonic beam: R2PI-TOF spectroscopy of diastereomeric carboxylic esters/(R)-(+)-1-phenyl-1-propanol complexes

Wavelength and mass resolved resonance-enhanced two photon ionization (R2PI) excitation spectra of (R)-(+)-1-phenyl-1-propanol (P(R)) and its complexes with some chiral esters, i.e. methyl lactates (L(R) and L(S)), methyl 3-hydroxybutyrates (H(R) and H(S)), and methyl 2-chloropropionates (C(R) and C(S)), have been recorded after a supersonic molecular beam expansion and interpreted in the light of DFT calculations. The spectral features of the selected complexes were found to depend on the nature of hydrogen-bond interactions within the diasteromeric complexes, whose intensity in turn depends upon the structure and the configuration of the estereal moiety. The study further confirms resonant two-photon ionization spectroscopy, coupled with time-of-flight mass resolution (R2PI-TOF), as an excellent tool for gathering valuable information on the interactive forces in molecular clusters and for the enantiodiscrimination of chiral molecules in the gas phase.

An axial molecular-beam diode laser spectrometer

A mid-infrared tunable diode laser molecular-beam spectrometer for the purpose of trace gas sensing and the study of van der Waals complexes is described. The spectrometer employs a Herriott multipass cell with up to 72 passes. The sample gas is injected parallel to the optical axis through a hole at the center of the far mirror. The molecular absorption is Doppler split, resulting from the laser beam propagating parallel and antiparallel to the molecular-beam expansion. The axial expansion leads to narrower line widths and increased sensitivity, compared to the traditional vertical injection method, as a result of selective sampling of the central part of the molecular expansion with reduced Doppler broadening and longer residence time of the molecular sample in the laser beam. The molecular expansion leads also to selective signal enhancement of low-J transitions, as demonstrated for the ν3 antisymmetric stretch vibration of CO2. A microwave horn antenna was implemented into the spectrometer to enable microwave–infrared double-resonance experiments. The spectrometer performance was evaluated by recording spectra of the CO2–Ar, (CO2)2, CO2–He, and CO2–SO2 van der Waals complexes near the R(0) transition of the ν3 band of CO2 around 2349 cm−1. The feasibility of using a pulsed molecular expansion for trace gas sensing is explored.

Chiral recognition of diols by complexation with (R)-(+)-1-phenyl-1-propanol: a R2PI approach in supersonic beam

Wavelength and mass resolved resonance-enhanced multiphoton ionization (REMPI) excitation spectra of (R)-(+)-1-phenyl-1-propanol (PR) and its complexes with some chiral diols, i.e. 1,2-propanediols, 2,3-butanediols, and 2,4-pentanediols, have been recorded after a supersonic molecular beam expansion and interpreted in the light of molecular dynamic (MD) conformational minima searches. The spectral features of the selected complexes were found to depend on cooperative hydrogen-bond interactions between the two components, whose intensity depends upon the specific configuration of the diol moiety and the relative position of its hydroxy groups. The study further confirms resonant two-photon ionization spectroscopy, coupled with time-of-flight mass resolution (R2PI-TOF), as an excellent tool for gathering valuable information on the interactive forces in molecular clusters and for enantiodiscrimination of chiral molecules in the gas phase.

Laser spectroscopy of 1,2,3,4-tetrahydro-1-naphthol·ROH (R = H, CH3) clustersPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

Wavelength and mass selected resonant enhanced multi-photon ionization (REMPI) spectroscopy is an excellent tool for characterising molecular clusters. Mass selected resonant two photon ionisation (R2PI) spectra of the S1 ← S0 transition of 1,2,3,4-tetrahydro-1-naphthol (TR) and their complexes with solvent molecules such as water (W) and methanol (MeOH) have been recorded after a supersonic molecular beam expansion. Spectra have been analysed with the aid of ab initio molecular orbital calculations conducted at the B3LYP/6-31G** level of the theory.

Investigation of Conformationally Rich Molecules: Rotational Spectra of Fifteen Conformational Isomers of 1-Octene

The rotational spectra of 15 conformational isomers of 1-octene have been measured in a molecular beam at a rotational temperature of less than 2 K using a pulsed-nozzle Fourier transform microwave spectrometer. The transition assignments are guided by rotational constant calculations based on moderate-level ab initio theory and on the relative energy minima on the MM3 molecular-mechanics force field. The number of conformers identified is slightly more than 10% of the 131 predicted from the MM3 molecular-mechanics force field of Allinger et al. Fourteen of the observed conformers are identified with 14 of the 15 lowest energy minima predicted from the MM3 molecular-mechanics force field. The observation of such a large number of conformers, with a MM3 calculated energy spread of 365 cm-1 for this subset of 14, is a consequence of the minimal conformational cooling in the molecular-beam expansion. Limiting this cooling are the relatively high barriers separating the conformers compared to the thermal energy, kT, of the preexpansion gas. In some cases, primarily for the higher energy conformers, the need to cross multiple internal-rotation barriers provides an additional bottleneck for conformer relaxation. The rotational spectra furnish values for the principal moments of inertia, which are sensitive to the conformational geometry and can be compared with predictions from future high-level ab initio calculations. Assuming no conformer relaxation in the expansion allows the use of the transition-intensity data to estimate the energy ordering of the conformers.

Photochemical R2PI study of chirality and intermolecular forces in supersonic beam

One and two‐color, mass selected R2PI spectra of the S1 ← S0 transitions in the bare (+)-(R)- 1-phenyl-1-ethanol (ER) and its complexes with different solvent molecules (solv) (‐)‐(R)-2-butanol (BR) or (+)-(S)-2-butanol (BS), (—)‐(R)-2-pentanol (TR) or (+)-(S)-2-pentanol ( TS) and (-)-(R)-2-butylamine (AR) or (+)-(S)-2-butylamine (AS), have been recorded after a supersonic molecular beam expansion. The one‐color R2PI excitation spectra of the diastereomeric complexes are characterized by significant shifts of their band origin relative to that of bare ER. The extent and the direction of these spectral shifts are found to depend upon the structure and the configuration of solv and are attributed to different short‐range interactions in the ground and excited states of the complexes. In analogy with other diastereomeric complexes, the phenomenological binding energy of the homochiral cluster is found to be greater than that of the heterochiral one. Preliminary measurements of excitation spectrum of (+)-(R)-1-Indanol (IR) is also reported.

R2PI Study of intermolecular hydrogen bond in solvent-free chiral complexes.

One- and two-color, mass selected R2PI spectra of the S(1)<--S(0) transitions in the bare (+)-(R)-1-phenyl-1-ethanol (E(R)) and its complexes with different solvent molecules (solv) (-)-(R)-2-butanol (B(R)) or (+)-(S)-2-butanol (B(S)) and (-)-(R)-2-butylamine (A(R)) or (+)-(S)-2-butylamine (A(S)), have been recorded after a supersonic molecular beam expansion. The one-color R2PI excitation spectra of the diastereomeric complexes are characterized by significant shifts of their band origin relative to that of bare E(R). The extent and the direction of these spectral shifts are found to depend on the structure and the configuration of solv and are attributed to different short-range interactions in the ground and excited states of the complexes. In analogy with other diastereomeric complexes, the phenomenological binding energy of the homochiral cluster is found to be greater than that of the heterochiral one.

Gas-phase measurement of Δ H0 between axial and equatorial conformations of 3-methylcyclopentanone

The energy difference between axial-methyl and equatorial-methyl isomers of 3-methylcyclopentanone (3MCPO) was measured for the first time using resonance-enhanced multiphoton ionization spectroscopy. Variable temperature molecule beam spectroscopy was used to determine a Δ H 0 of 1.19±0.14 kcal/mol which is considerably lower than the corresponding value for 3-methylcyclohexanone. This is because the 1,3-diaxial interaction between the methyl group and hydrogen atom is smaller in 3MCPO. The transition state for interconversion between the isomers of 3MCPO is calculated to lie 1.4 kcal/mol above the axial conformer energy. In spite of this small activation energy, the van’t Hoff plot shows that vibrational cooling of the conformers is much more efficient than isomerization during the supersonic expansion up to the highest temperature investigated. This means that in the molecular beam expansion, the two conformers are frozen out instantaneously.

Short-range interactions within molecular complexes formed in supersonic beams: structural effects and chiral discrimination

One- and two-color, mass-selected R2PI spectra of the S1<--S0 transitions in the bare chiral chromophore R-(+)-1-phenyl-1-propanol (R) and its complexes with a variety of alcoholic solvent molecules (solv), namely methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, S-(+)-2-butanol, R-(-)-2-butanol, 1-pentanol, S-(+)-2-pentanol, R-(-)-2-pentanol, and 3-pentanol, were recorded after a supersonic molecular beam expansion. Spectral analysis, coupled with theoretical calculations, indicate that several hydrogen-bonded [R.solv] conformers are present in the beam. The R2PI excitation spectra of [R.solv] are characterized by significant shifts of their band origin relative to that of bare R. The extent and direction of these spectral shifts depend on the structure and configuration of solv and are attributed to different short-range interactions in the ground and excited [R.solv] complexes. Measurement of the binding energies of [R.solv] in their neutral and ionic states points to a subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces, which control the spectral features of the complexes and allow enantiomeric discrimination of chiral solv molecules.

Chirality and intermolecular forces: studies using R2PI experiments in supersonic beams

One- and two-color, mass-selected resonant two-photon ionization (R2PI) spectra of the S1←S0 transitions in the bare (R)-(+)-1-phenylethan-1-ol (ER) and its complexes with a solvent molecule (solv: (S)-(+)butan-2-ol (BS), (R)-(−)butan-2-ol (BR), or water (W)) have been recorded after a supersonic molecular beam expansion. The excitation spectrum of bare ER conforms to theoretical predictions at the B3LYP/6-31G** level of theory by pointing to the formation of a single conformer. The one-color R2PI excitation spectra of the diastereomeric complexes [ER–solv] (solv: BS or BR) are characterized by significant shifts of their band origin relative to that of bare ER. The extent and the direction of these spectral shifts are found to depend upon the structure and the configuration of solv and are attributed to different short-range interactions in the ground and excited [ER–solv] complexes. In analogy with strictly related diastereomeric complexes, the phenomenological binding energy of the homochiral [ER–BR] is found to be greater that of the heterochiral one [ER–BS]. The one-color R2PI excitation spectra of the [ER–W] complex displays two signals blue shifted by 54 and 73 cm−1, relative to the S1←S0 band origin of bare ER, which indicate the presence of a O–H···π electrostatic interaction between ER and W.

Rapid Evaporative Cooling Suppresses Fragmentation in Mass Spectrometry: Synthesis of “Unprotonated” Water Cluster Ions

Hydrogen-bonded water clusters were formed with inert gases adsorbed to them in a strong molecular beam expansion. Upon single-photon ionization of such mixed clusters using VUV light, fragmentation of the substrate water cluster ion is markedly suppressed. Experimental evidence is presented, showing that the rapid evaporation of the inert gas from the newly formed water cluster ion efficiently removes internal energy on a time scale much faster than the usual fragmentation reactions present in pure water clusters, i.e., rates of fragmentation that are normally >109 s-1. This phenomenon is exploited to produce “unprotonated” water clusters, formally (H2O)n+. Using post source decay reflectron time-of-flight mass spectrometry, the structure of the “unprotonated” water cluster ions is experimentally determined for the first time. The structure determined, H3O+(H2O)k·OH where the hydroxyl radical is found outside the first solvation shell of the charge, is consistent with recent ab initio calculations. This ...

Enhanced reactivity of pseudomorphic Co on Cu(111)

The reactivity of Co deposited on Cu(111) is determined using the dissociation of CH4 as a test reaction. The CH4 molecules are produced in a supersonic molecular beam expansion and dissociate on Co but not on Cu under the experimental conditions used here. It is found that the dissociation probability of CH4 on a few layers of pseudomorphic Co adsorbed on noble Cu(111) is larger than that of pure Co. The apparent CO binding energy is, however, not shifted as measured using thermal desorption. A model of this surprising behavior is presented using information on the structure and theoretical calculations on the shift of the d-band of Co adsorbed on Cu(111).

Ethyl loss kinetics of tetraethylsilane ions studied by TPEPICO

Because of the many low-frequency vibrational modes in tetraethylsilane and the apparent difficulty of vibrationally cooling it in a molecular beam expansion, the investigation of the dissociation dynamics of “energy-selected” tetraethylsilane ions is more complicated than for most smaller ions. By applying an analysis which takes into account the broad reactant internal energy distribution, the 0 K ethyl-loss activation energy and rate constant function of internal energy were derived from threshold photoelectron-photoion coincidence (TPEPICO) data. The dissociation transition state (TS) is found to be highly variational, with the bottleneck becoming increasingly entropic with increasing internal energy. Simulations of the breakdown diagram and individual time-of-flight (TOF) mass spectra are described in detail. The measured 0 K activation energy is 0.82 (0.05 eV, leading to a 0 K heat of formation for the product ion Si(C 2H 5) 3 + of 610 (16 kJ mol −1).

Resonant two-photon ionisation spectroscopy of $C_{60}$

The electronic absorption spectrum of C60 has been measured using a continuous gas phase source which is capable of cooling the molecules to a vibrational temperature below ca. 100 K. The results are in good agreement with a previous high resolution gas phase spectrum reported for the spectral range 595 nm–630 nm, obtained with a pulsed Smalley source [1]. The spectral range down to 450 nm is reported for the first time for a molecular beam expansion and is compared with published spectra obtained in solution [2].

Adsorption of NxOy-Based Molecules on Large Water Clusters: An Experimental and Theoretical Study

The sticking efficiencies for a range of NxOy species (NO, NO2, N2O, N2O5, and HNO3) onto large water clusters have been studied using a supersonic molecular beam expansion to generate water clusters containing 50−450 water molecules. NO, NO2, and N2O are found to stick with very low efficiencies relative to the organic species studied previously. This is in accord with studies for the corresponding species performed on cooled water ice films. In contrast, N2O5 is efficiently converted to nitric acid by a heterogeneous surface reaction with the water cluster. Quantum mechanical−molecular dynamics calculations of the energies for the interaction of NO, NO2 and N2O with a water molecule and a small water cluster, (H2O)10, have been performed. These show that the NOx molecule prefers to be bonded to the surface of the cluster and that it boils off the cluster at 140 K.

Adsorption of Organic Molecules on Large Water Clusters

The sticking efficiencies for a range of organic molecules (methanol, propan-2-ol, acetone, acetaldehyde, formic acid, benzene, trimethylamine, and ethene) onto large water clusters, containing several hundred water molecules, have been determined using a supersonic molecular beam expansion to generate the water clusters and the pickup technique to deposit the organic molecule onto the cluster. It is believed that the organic molecule remains adsorbed onto the surface of the cluster and is not incorporated into the bulk of the cluster. The relative sticking coefficients are found to correlate with the magnitudes of the interaction energy for the organic molecule and water. It is suggested that the higher sticking efficiencies (e.g., for formic acid and acetaldehyde) may result from the molecules forming more than one hydrogen bond to the water cluster.

Adsorption of Some Atmospherically Important Molecules onto Large Water Clusters: SO2, SO3, HCl, and ClONO2

AbstractThe adsorption of a range of atmospherically important molecules (SO2, SO3, HCl, and ClONO2) on large water clusters have been studied using a supersonic molecular beam expansion to generate water clusters containing 50–450 water molecules. SO2 and HCl were found to stick with low efficiency to the water cluster, retaining their chemical identity. In contrast, both SO3 and ClONO2 undergo heterogeneous reactions on the surface of the water cluster forming, respectively, H2SO4 and HOCl with HNO3. For SO3, calculations show that the large barrier that exists in the gas‐phase to the reaction of SO3 with water is removed for water clusters of a sufficient size because of stabilization of the transition state by solvation. For ClONO2, the barrier to reaction is much larger and cannot be removed by solvation for any size cluster. In this case, it is likely that reaction takes place on the water cluster by the ionic dissociation of ClONO2 in a similar manner to that observed for ClONO2 on ice films.

Formation, Structure, and Stabilities of Metallocarbohedrenes

Time-of-flight mass spectrometric investigations of the formation of metallocarbohedrenes and binary metal metallocarbohedrenes under both direct laser vaporization and laser vaporization/molecular beam conditions were conducted. A wide range of Ti-to-carbon ratios and in other experiments a range of Ti-to-Zr molar ratios were explored. These new studies provide further insight into the formation process of Met-Car clusters and, moreover, serve to eliminate certain suggested formation mechanisms such as one involving the attachment of metal atoms to a preexisting core consisting of a 12-atom carbon cage. There is no evidence in the experiments to suggest that Met-Cars produced via direct laser vaporization exist in a higher energy configuration than those formed in a laser plasma and then subjected to cooling in a molecular beam expansion. Mass spectral distributions of binary metal metallocarbohedrenes produced under widely varying conditions provide evidence that there is one set of equivalent metal sit...

Laser assisted molecular beam deposition of thin films of polymeric copperphthalocyanine and their characterization

AbstractThin films of polymeric copperphthalocyanine were fabricated via laser assisted molecular beam deposition by reacting the laser ablated plasma flume of a copper target with 1,2,3,5-tetracyanobenzene entrained within a molecular beam expansion. 1,2,4,5-Tetracyanobenzene entrained within a helium carrier gas was supersonically expanded to intercept a second expansion containing gas-phase aggregates of metallic copper. These two molecular beams met at a point between the target and the substrate producing gas phase polymeric copperphthalocyanine which could be then directly deposited as a thin film coating on room temperature substrates. Such films were characterized by scanning electron microscopy and uv-visible, infrared and x-ray photoelectron spectroscopic techniques. Optical spectra of films dissolved in H2S04 solutions reveal that small amounts of monomeric copperphthalocyanine is also present in the grown films. All the copper in these films is found to be in Cu+2 ionization state.

Calibration of Total Pressure Gauges in the UHV and XHV Regions.

When total pressures decrease to the XHV region, measurements with ionization gauges become increasingly sensitive to disturbances such as electron- or photonstimulated desorption, outgassing of the gauges, and production of secondary electrons by X-rays. Therefore, it is very important to have a reliable pressure scale also in the XHV region. The laboratory for vacuum physics at the Physikalisch-Tech-nische Bundesanstalt (PTB) in Germany provides a calibration system with molecular beam expansion for pressures down to 10-10 Pa and is building a new calibration system based on continuous expansion for a lower calibration limit in the 10-11 Pa decade. The advantages and disadvantages of the two systems will be discussed and calibrations with the molecular beam system will be presented. For determining the outgassing rate of the vacuum chamber of the new calibration system, a new method was applied and compared to the classical methods, the throughput method and the pressure rise method. In particular it was checked whether the outgassing rate depends on the pumping speed applied to the chamber as proposed in a theory by Malev.