Electrostatic Hexapole Field Research Articles

Electrostatic Hexapole State-Selection of the Asymmetric-Top Molecule Propylene Oxide

Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.

Negative collision energy dependence of Br formation in the OH + HBr reaction

The reaction between HBr and OH leading to H(2)O and Br in its ground state is studied by means of a crossed molecular beam experiment for a collision energy varying from 0.05 to 0.26 eV, the initial OH being selected in the state |JOmega> = |3/2 3/2> by an electrostatic hexapole field. The reaction cross-section is found to decrease with increasing collision energy. This negative dependence suggests that there is no barrier on the potential energy surface for the formation pathway considered. The experimental results are compared with the previously reported quantum scattering calculations of Clary et al. (D. C. Clary, G. Nyman and R. Hernandez, J. Phys. Chem., 1994, 101, 3704), and briefly discussed in the light of skewed potential energy surfaces associated with heavy-light-heavy type reactions.

Development of a highly intense state-selected CH radical beam source for the study on the collision energy dependent reaction dynamics

A velocity-variable highly intense pulsed supersonic CH (X 2Π) radical beam source was newly developed for the study on the collision energy dependent reaction dynamics of the state-selected CH radical. CH radicals were state-selected by an electrostatic hexapole field. The focusing curves and the CH flux intensities for the ∣ J, F i, M〉 states were measured by a saturated laser-induced fluorescence spectroscopy. As a demonstration, the rotational state dependence of the reaction cross section of CH + O 2 → OH(A) + CO reaction was determined at the collision energy of 0.06 eV.

State-selection and orientation of CH radicals by an electric hexapole

A pure and high intense pulsed supersonic CH (X2Π ) radical beam source was developed via the C(1D) + H2 reaction. An electrostatic hexapole field was used to state-select CH radicals. The focusing curves for the single rotational states of CH were measured for the first time by a saturated laser-induced fluorescence (SLIF) spectroscopy for the R-branch in A2Δ3/2 ← X2Π 1/2 transition. The focusing curves were simulated by the classical trajectory simulation based on a Stark energy analysis of the rotational energy levels, including spin-orbit and Λ-doubling coupling effects. In addition, orientational distribution functions were calculated for the selectable states.

Rotational state-resolved reaction cross section in the reactions of state-selected CH with NO and with O2

A pure and highly intense state-selected pulsed supersonic CH(X (2)Pi) radical beam source was developed by use of the C((1)D)+H(2) reaction with the combination of the state selection and purification by an electrostatic hexapole field. Under the beam-cell condition, the elementary reactions of CH+NO and CH+O(2) were studied by using this state-selected CH beam. NH(A (3)Pi) [and NCO(A (2)Sigma(+))] formations and OH(A (2)Sigma(+)) formation were directly identified in the elementary reaction of CH+NO and CH+O(2), respectively. For the CH+NO reaction, the relative branching ratio sigma(NCO*)sigma(NH) of NCO(A (2)Sigma(+)) formation to NH(A (3)Pi) formation was determined to be 0.35+/-0.15. The state-selected reaction cross sections were determined for each rotational state of CH. In the CH+NO reaction, a remarkable rotational state dependence of the reactive cross section was revealed, while the CH+O(2) reaction showed little rotational state dependence.

A highly intense state-selected CH radical beam and its application to the CH + O 2 reaction

A highly intense pulsed supersonic CH(X 2Π) radical beam source was developed by use of the C( 1D) + H 2 reaction. CH radicals were state-selected by an electrostatic hexapole field. The focusing curves for the single state-selected CH were measured for the first time by a saturated laser-induced fluorescence (LIF) spectroscopy for the R-branch in A 2Δ 3/2 ← X 2Π 1/2 transition. The maximum CH flux intensity in the lowest ∣ J, F, M〉 state, ∣1/2, 2, 1/2〉 was measured by LIF to be 3.2 × 10 12 rad cm −2 s −1 at the focusing point of 150 cm downstream from the nozzle.

Structures and its dipole moments of half-sandwich type metal–benzene (1:1) complexes determined by 2-m long electrostatic hexapole

The transition metal–benzene (1:1) complexes are synthesized by laser evaporation method followed by the size and structure selection with 2-m long electrostatic hexapole field. Electric dipole moments of half-sandwich type binary complexes for a series of the transition metals, i.e. Ti, V, Co, and Ni, with benzene molecule were determined by measuring the focusing curves, namely the dependence of the focused beam intensity upon the hexapole field strength. It is found that the dipole moments of Ti–C6H6 and V–C6H6 are larger than the ones of Co–C6H6 and Ni–C6H6. Such large dipole moments of Ti–C6H6 and V–C6H6 can be ascribed to the strong orbital interaction due to charge transfer between the metal atom and benzene molecule.

A highly intense state-selected OH beam source by the pulsed electric DC discharge method

A high intensity pulsed DC discharge source for OH radical beam was developed by the systematic optimization for geometry of the discharge source and for the operation conditions. OH radicals produced in the DC discharge source were state-selected by an electrostatic hexapole field. The maximum OH flux in the lowest | J, Ω, M〉=|3/2,3/2,3/2〉 state was directly measured to be 8.5 × 10 14 radicals cm −2 s −1 at the beam crossing point of 150 cm downstream from nozzle by a saturated laser-induced fluorescence spectroscopy for the Q 1(1) transition in A 2Σ←X 2Π 3/2 transition.

Comparison of OH(A–X) chemiluminescence spectra from photo-initiated intra-cluster reaction of HBr–N 2O isomers prepared with He and Ar seeding gases

The internal energy distribution of product OH(A) was measured for the photo-initiated intra-cluster reaction of HBr–N 2O prepared using either Ar or He seeding gas. We find that rotation of OH(A) produced from HBr–N 2O seeded in Ar is more excited than that from HBr–N 2O seeded in He. This difference could be ascribed to the difference in relative abundance of HBr–N 2O geometric isomers in the cluster beam. The present result is consistent with the previous experimental finding in the focusing experiment of HBr–N 2O using the electrostatic hexapole field, namely the preferential formation of the linear isomer under Ar seeding condition.

Metal–ligand interaction of Ti–C 6H 6 complex size-selected by a 2-m long electrostatic hexapole field

Supersonic beam of size-selected Ti–C 6H 6 metal–ligand binary complex was generated by a laser evaporation method followed by the size selection with 2-m long electrostatic hexapole field. The dependence of the beam intensity upon the hexapole field strength was measured in order to determine permanent dipole moment and complex symmetry. We find that the structure of Ti–C 6H 6 complex has nearly C 6v symmetry and its electric dipole moment is 2.4±0.3 D. The calculation with use of density functional theory substantiated its optimized structure as C 6v symmetric in accordance with the experimental result. The calculation also suggests that charge transfer interaction between Ti atom and C 6H 6 plays a central role to stabilize the complex.

Focusing and selecting the linear type HBr–N2O by using a 2 m long electrostatic hexapole field

Focusing and selecting the HBr–N2O cluster beam was performed by using a 2 m long electrostatic hexapole field. The observed focusing curve shows a clear evidence that the linear type HBr–N2O isomer which has symmetry of symmetric top was preferentially detected in the cluster beam formed under the experimental condition used here, even though our previous ab initio calculations predict that the bent type isomer is theoretically possible. The best fit simulation for the experimental focusing curve was achieved only if we assume vibrational excitation in the van der Waals mode of the linear type HBr–N2O. The permanent dipole moment is determined to be 0.50±0.05 D.

Evidence for the HCl+(A) Formation in the Reaction of Ne(3P) with the Size-Selected HCl Dimer Using an Electrostatic Hexapole Field

Abstract Penning ionization of Ne(3P) metastable atom with size-selected HCl dimer was studied by using an electrostatic hexapole field and chemiluminescence detection. We found that the HCl dimer can also produce HCl+(A) ions in the reaction with Ne(3P) just like its corresponding monomer reaction. The internal energy distribution of HCl+(A) in the dimer reaction is cooler than that in the monomer reaction, reflecting the third-body effect due to the other HCl in the dimer.

Direct determination of the permanent dipole moments and structures of Al-CH(3)CN and Al-NH(3) by using a 2-m electrostatic hexapole field.

The supersonic beams of the (1-1) metal-ligand complexes of Al-CH(3)CN and Al-NH(3) were produced by a laser evaporation method. Nondestructive structure selection of the complexes and the dipole moment determination were performed by using a 2-m electrostatic hexapole field. The experimentally determined permanent dipole moments are 1.2 +/- 0.1 D for Al-CH(3)CN and 2.7 +/- 0.2 D for Al-NH(3). We find that the dipole moment of Al-NH(3) becomes larger than that of neat NH(3), while the formation of the Al-CH(3)CN complex produces a smaller dipole moment than that of neat CH(3)CN on the other hand. We performed the ab initio calculations to draw out plausible complex structures and to clarify the bonding character after formation of the complex, and we made comparisons with the computational results done by several groups. The Mulliken population analysis suggests the Al-->CH(3)CN charge flow, but on the other hand the Natural population analysis indicates very little charge flow. For the Al-NH(3) complex, the polarization effect of NH(3) and the N-->Al sigma donation would enhance the dipole moment strength. However, there still remains a controversial disagreement between the theoretical predictions and the experimental results. Further experimental determination using the hexapole method for various metal-ligand complexes and clusters could reveal the basic nature of interaction in the complex systems in general, and this method would complement theoretical calculations.

Non-destructive selection of geometrical isomers of the Al(C6H6) cluster by a 2 m electrostatic hexapole field

A supersonic cluster beam containing isomers of Al(C6H6) complexes was generated by laser evaporation, and non-destructively selected using a 2 m long electrostatic hexapole. The focusing curve shows clear evidence that there are two kinds of Al(C6H6) isomers which are slightly different from each other in geometry; one is an asymmetric 1,2-complex and the other nearly C6v symmetric 1,4-complex. The electric dipole moments of the two isomers are found to be 1.5 ± 0.1 and 1.4 ± 0.1 D, respectively. We carried out computation using density functional theory in order to estimate their structures. We found that the 1,2-complex is more stable than the 1,4-complex. The present work confirms that the electrostatic hexapole technique is useful for non-destructive selection of geometrical isomers in a beam.

Photodissociation of DCl dimer selected by an electrostatic hexapole field combined with a Doppler-selected time-of-flight technique: observation of [ClDCl] transient species

The photodissociation of the DCl dimer, which was preferentially selected from the cluster beam using a hexapole electrostatic field prior to the photolysis, was studied by a Doppler-selected time-of-flight (DS-TOF) technique at 243 nm. We observed the [ClDCl] transient product after deuterium elimination from (DCl)2. By measuring the dependence of the enhanced signal for the photodissociated D atom on the hexapole voltage, it was found that the DS-TOF spectrum is composed of two kinds of velocity component: one is a fast velocity component which originates from the dimer only, and the other is a slow velocity component which originates not only from the dimer but also from higher DCl cluster sizes. The fast velocity component of the spectrum shows oscillating structures, which may be an indication of the nascent internal (mainly vibrational) state of [ClDCl]. The spacing of the peaks is about 800 cm−1, which is less than half of the normal stretching frequency of the DCl monomer (i.e. 2091 cm−1). Therefore, we tentatively conclude that the present spectrum exhibits strong perturbation by the adjacent Cl atom in the [ClDCl] transient species.

Tunneling motion in (HCl)2 hydrogen-bonded dimer probed by electrostatic hexapole and Doppler-selected TOF measurement for the internal energy distribution of [ClHCl

The tunneling motion in (HCl)2 hydrogen bonded dimer and its deuterate was probed by a 2m long electrostatic hexapole field. The focusing curves of the dimers confirmed the existence of homo and heterodimers in the cluster beam. The homodimer, either H35Cl–H35Cl or H37Cl–H37Cl, undergoes a fast tunneling motion for the two hydrogen atoms in the dimer. The heterodimer, namely H35Cl–H37Cl, on the other hand, does not show such fast tunneling motion in the time scale of the experiment. The electric dipole moments for both (DCl)2 isotopomers were determined to be 1.5±0.2D, which is the same value for (HCl)2. The observed ratio of homo to heterodimer was estimated to be 30±10, and this value differs largely from the natural abundance for the chlorine isotope.An experimental scheme to discern homo and heterodimers is proposed here. By looking at fragments in the (HCl)2 dimer photodissociation using a Doppler-selected time-of-flight (TOF) technique, internal energy distribution of the [ClHCl] fragment was measured in 121.6nm photodissociation. The TOF spectrum consists of fast and slow velocity components for the dissociated H atoms. It is found that the slow H component that arises from the hydrogen escapes after many collisions. The fast H component that arises from the direct H escape without any collision, thus this component reflects an internal and/or electronic state of the counter part fragment, i.e. [ClHCl]. The vibrational structure of [ClHCl] was observed for the fast H component of the TOF spectrum.

Trapping of neutral molecules in a single state by a combination of electrostatic and dynamic fields

In this paper, we show that it is possible to optically trap, in two or three dimensions, a molecule oriented in space in a single | JKM〉 rotational state of its ground electronic motion. It is readily achieved by shining a far-off resonance laser of moderate intensity on the molecule in a weak, homogenous, orienting electric field E → 0 preceded by an electrostatic hexapole field. The harmonic trapping force depends upon, among other things, the orientation of the laser with respect to E → 0 and the state | JKM〉 of the molecule, in addition to molecular dynamical polarizability.

Focusing of DCl and HCl dimers by an electrostatic hexapole field: The role of the tunneling motion

The focusing of HCl and DCl dimers was observed using a 2-m-long electrostatic hexapole field. The results indicate the existence of two types of species. The first is the homodimers, either the H35Cl–H35Cl or the D35Cl–D35Cl, for which the data indicate a fast tunneling motion. The second is the heterodimers, H35Cl–H37Cl or D35Cl–D37Cl, that do not show evidence for significant tunneling motion on the time scale of the experiment. In the case of HCl dimers, even at relatively high fields, only one species could be focused, the heterodimer. The electric dipole moments for both (DCl)2 isotopomers were determined to be 1.5±0.2 D, which is the same value as observed for (HCl)2.

Rotationally state-selected and oriented molecules: Photoelectron spectroscopy and chirality of C3v molecules

This paper reports a theoretical study of photoionization in a gaseous C 3v molecule prepared in a single rotational state of its ground electronic motion by an electrostatic hexapole field and oriented in space by the subsequent application of a weak homogeneous electric field . Markedly different cross-sections are calculated for electrons ejected from 13a2 1 orbital of a molecule by electromagnetic radiation plane polarized parallel and perpendicular to . These photocurrents change further in different manners with rotational states of . In accord with the experimental observation by Kaesdorf [Phys. Rev. Lett. 54, 885 (1985)], we also find a pronounced asymmetry in the photocurrents emitted in directions along and opposite to . This forward - backward asymmetry, which very strongly depends upon the rotational state of the molecule, is however more pronounced in parallel than in perpendicular experimental geometry. Both circular as well as linear dichroisms have also been studied, but neither of these shows any forward-backward asymmetry in the present application.

Experimental determination of the dipole moment of HCl dimer using an electrostatic hexapole field

HCl dimers were prepared in a pulsed supersonic beam of neat HCl. The dimers were focused in an electrostatic hexapole field. Using computer simulation, the focusing curve could be fit, assuming contributions from both first and second order Stark effects. The dipole moment, based on simulation, was found to be 1.5 ± 0.1 D. The ability to observe an effective dipole moment in a system where the donor-acceptor tunneling is fast is surprising. It may indicate that the understanding of the interaction of floppy systems with hexapole field is not complete.