Pendular state spectroscopy of an asymmetric top: Parallel and perpendicular bands of acetylene-HF

Controlling Protein Orientation in Vacuum Using Electric Fields

Single-particle imaging using X-ray free-electron lasers is an emerging technique that could provide high-resolution structures of macromolecules in the gas phase. One of the largest difficulties in realizing this goal is the unknown orientation of the individual sample molecules at the time of exposure. Preorientation of the molecules has been identified as a possible solution to this problem. Using molecular dynamics simulations, we identify a range of electric field strengths where proteins become oriented without losing their structure. For a number of experimentally relevant cases we show that structure determination is possible only when orientation information is included in the orientation-recovery process. We conclude that nondestructive field orientation of intact proteins is feasible and that it enables a range of new structural investigations with single-particle imaging.

Dependences of Q-branch integrated intensity of linear-molecule pendular spectra on electric-field strength and rotational temperature and its potential applications

We calculate the pendular-state spectra of cold linear molecules, and investigated the dependences of “Q-branch” integrated intensity of pendular spectra on both electric-field strength and molecular rotation-temperature. A new multi-peak structure in the “Q-branch” spectrum is appearing when the Stark interaction strength ω = μE/B equal to or larger than the critical value. Our study shows that the above results can be used not only to measure the electric-field vector and its spatial distribution in some electrostatic devices, such as the Stark decelerator, Stark velocity filter and electrostatic trap and so on, but also to survey the orientation degree of cold linear molecules in a strong electrostatic field.

Ultrafast angular momentum orientation by linearly polarized laser fields

We theoretically show and experimentally verify that a pair of linearly polarized intense femtosecond pulses can create molecular ensembles with oriented rotational angular momentum on an ultrafast (approximately ps) time scale, when the delay and the mutual polarization between them are appropriately arranged. An asymmetric distribution for +M and -M sublevels relies on quantum interference between rotational wave packets created in stimulated Raman excitation by the first and second pulses. The present approach provides spatiotemporally propagating ensembles, of which the classical perspective is molecules rotating in a clockwise or counterclockwise direction.

Photofragment Spectroscopy and Predissociation Dynamics of Weakly Bound Molecules

Photofragment spectroscopy is combined with imaging techniques and time-resolved measurements of photoions and photoelectrons to explore the predissociation dynamics of weakly bound molecules. Recent experimental advances include measurements of pair-correlated distributions, in which energy disposal in one cofragment is correlated with a state-selected level of the other fragment, and femtosecond pump-probe experiments, in some cases with coincidence detection. An application in which coincident measurements are carried out in the molecular frame is also described. To illustrate these state-selective and time-resolved techniques, we review two recent applications: (a) the photoinitiated dissociation of the covalently bound NO dimer on the ground and excited electronic states and the role of state couplings and (b) the state-selected vibrational predissociation of hydrogen-bonded acetylene dimers with HCl (acid) and ammonia (base) and the importance of angular momentum constraints. We highlight the crucial role of theoretical models in interpreting results.

Infrared Spectroscopy of Prereactive Aluminum−, Gallium−, and Indium−HCN Entrance Channel Complexes Solvated in Helium Nanodroplets

Prereactive metal atom-HCN entrance channel complexes [M-HCN (M=Al, Ga, In)] have been stabilized in helium nanodroplets. Rotationally resolved infrared spectra are reported for the CH stretching vibration of the linear nitrogen-bound HCN-Ga and HCN-In complexes that show significant perturbation due to spin-orbit coupling of the 2Pi1/2 ground state with the 2Sigma1/2 state which are degenerate at long range. Six unresolved bands are also observed and assigned to the linear hydrogen-bound isomers of Al-HCN, Ga-HCN, and In-HCN corresponding to the fundamental CH stretching vibration and a combination band involving the CH stretch plus intermolecular stretch for each isomer. A nitrogen-bound HCN-Al complex is not observed, which is attributed to reaction, even at 0.37 K. This conclusion is supported by the observation of a weakly bound complex containing two HCN's and one Al atom which, from the analysis of its rotationally resolved zero-field and Stark spectra is assigned to a weakly bound complex of a HCNAl reaction product and a second HCN molecule. Theoretical calculations are presented to elucidate the reaction mechanisms and energetics of these metal atom reactions with HCN.

High-Resolution Infrared Spectroscopy of HCN−Agn (n = 1−4) Complexes Solvated in Superfluid Helium Droplets

High-resolution infrared spectroscopy has been used to determine the structures, C-H stretching frequencies, and dipole moments of the HCN-Agn (n = 1-3) complexes formed in superfluid helium droplets. The HCN-Ag4 cluster was tentatively assigned based upon pick-up cell pressure dependencies and harmonic vibrational shift calculations. Ab initio and density functional theory calculations were used in conjunction with the high-resolution spectra to analyze the bonding nature of each cluster. All monoligated species reported here are bound through the nitrogen end of the HCN molecule. The HCN-Agn complexes are structurally similar to the previously reported HCN-Cun clusters, with the exception of the HCN-Ag binary complex. Although the interaction between the HCN and the Agn clusters follows the same trends as the HCN-Cun clusters, the more diffuse nature of the electrons surrounding the silver atoms results in a much weaker interaction.

Infrared Laser Spectroscopy of Uracil and Thymine in Helium Nanodroplets: Vibrational Transition Moment Angle Study

Vibrational spectra are reported in the N-H stretching region for uracil and thymine monomers in helium nanodroplets. Each monomer shows only a single isomer, the global minimum, in agreement with previous experimental and theoretical studies. The assignment of the infrared vibrational bands in the spectra is aided by the measurement of the corresponding vibrational transition moment angles (VTMAs) and ab initio frequency calculations. The ambiguity in the VTMA assignment of the N3H band for the uracil monomer is explained by the presence of dimer bands, which are overlapped with the monomer band.

Infrared Laser Spectroscopy of Imidazole Complexes in Helium Nanodroplets: Monomer, Dimer, and Binary Water Complexes

Infrared laser spectroscopy has been used to characterize imidazole (IM), imidazole dimer (IMD), and imidazole-water (IMW) binary systems formed in helium nanodroplets. The experimental results are compared with ab initio calculations reported here. Vibrational transition moment angles provide conclusive assignments for the various complexes studied here, including IM, one isomer of IMD, and two isomers of the IMW binary complexes.

Four Tautomers of Isolated Guanine from Infrared Laser Spectroscopy in Helium Nanodroplets

Infrared laser spectroscopy is used to study the four lowest energy tautomers of guanine, isolated in helium nanodroplets. The large number of vibrational bands observed in the infrared spectrum are assigned by comparing the corresponding experimental vibrational transition moment angles with those obtained from ab initio theory. The result is the conclusive assignment of the spectrum to the N9H-Keto, N7H-Keto, N9Ha-Enol(trans), and N9Hb-Enol(cis) tautomers. The dipole moments of these tautomers are also experimentally determined and compared with ab initio theory.

High-Resolution Infrared Spectroscopy of HCN−Znn (n = 1−4) Clusters: Structure Determination and Comparisons with Theory

High-resolution infrared laser spectroscopy has been used to obtain rotationally resolved spectra of HCN-Zn(n) (n = 1-4) complexes formed in helium nanodroplets. In the present study the droplets passed through a metal oven, where the zinc vapor pressure was adjusted until one or more atoms were captured by the droplets. A second pickup cell was then used to dope the droplets with a single HCN molecule. Rotationally resolved infrared spectra are obtained for all of these complexes, providing valuable information concerning their structures. Stark spectra are reported and used to determine the corresponding permanent electric dipole moments. Ab initio calculations are also reported for these complexes for comparison with the experimental results.

Infrared laser spectroscopy of CH3⋯HF in helium nanodroplets: The exit-channel complex of the F+CH4 reaction

High-resolution infrared laser spectroscopy is used to study the CH3...HF and CD3...HF radical complexes, corresponding to the exit-channel complex in the F + CH4 --> HF + CH3 reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v = 1 <-- 0 H-F vibrational band, the analysis of which reveals a complex with C(3v) symmetry. The vibrational band origin for the CH3...HF complex (3797.00 cm(-1)) is significantly redshifted from that of the HF monomer (3959.19 cm(-1)), consistent with the hydrogen-bonded structure predicted by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4+/-0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH3...HF to CD3...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F + CH4 surface, including geometry optimizations and vibrational frequency calculations for CH3...HF.

The isomers of HF–HCN formed in helium nanodroplets: Infrared spectroscopy and ab initio calculations

Binary complexes containing hydrogen cyanide and hydrogen fluoride are formed in helium nanodroplets, and studied using high-resolution infrared laser spectroscopy. Rotationally resolved spectra are reported for the H-F and C-H stretches of the linear HCN-HF complex, a system that has been thoroughly studied in the gas phase. We report the high-resolution spectra of the higher energy, bent HF-HCN isomer, which is also formed in helium. Stark spectra are reported for both isomers, providing dipole moments of these complexes. The experimental results are compared with ab initio calculations, also reported here. Spectra are reported for several ternary complexes, including (HCN)2-HF, HCN-(HF)2, HF-(HCN)2, and HF-HCN-HF.

Multiple isomers of uracil–water complexes: infrared spectroscopy in helium nanodroplets

Infrared laser spectroscopy is used to show that four structural isomers of the uracil-water binary complex are formed in helium nanodroplets. The assignment of the infrared spectra is aided by measurements of vibrational transition moment angles (VTMAs) for various vibrational modes of these complexes. The experimental results are in excellent agreement with ab initio calculations, which had previously predicated the existence of the same four isomers. The results suggest that the relative abundances of the various isomers formed in helium droplets have more to do with the widths of the valleys in the potential surface that funnel into a particular local minimum than on the associated energetics.

Pendular-state spectroscopy of the S1–S0 electronic transition of 9-cyanoanthracene

Fluorescence excitation spectra of the S(1)-S(0) origin band of 9-cyanoanthracene have been observed under a uniform electric field up to 200 kV/cm to explore pendular-state spectrum of an asymmetric-top molecule close to the strong field limit. The observed spectra exhibit distinct evolution of the band contour as a function of the applied electric field, which are much different from each other for different excitation configurations. An approximate method suitable for spectrum simulations of large asymmetric-top molecules in a pendular condition is developed for the analysis of the experimental results. The comparison of the observed and simulated spectra shows that the spectra are well ascribed in terms of the pendular-state selection rules, which have recently been derived from theoretical consideration of the pendular-limit representation of energy levels and spectra.

Infrared–infrared double resonance spectroscopy of cyanoacetylene in helium nanodroplets

Infrared-infrared double resonance spectroscopy is used as a probe of the vibrational dynamics of cyanoacetylene in helium droplets. The nu1 C-H stretching vibration of cyanoacetylene is excited by an infrared laser and subsequent vibrational relaxation results in the evaporation of approximately 660 helium atoms from the droplet. A second probe laser is then used to excite the same C-H stretching vibration downstream of the pump, corresponding to a time delay of approximately 175 micros. The hole burned by the pump laser is narrower than the single resonance spectrum, owing to the fact that the latter is inhomogeneously broadened by the droplet size distribution. The line width of the hole is characteristic of another broadening source that depends strongly on droplet size.

Polar isomer of formic acid dimers formed in helium nanodroplets

The infrared spectrum of formic acid dimers in helium nanodroplets has been observed corresponding to excitation of the "free" OH and CH stretches. The experimental results are consistent with a polar acyclic structure for the dimer. The formation of this structure in helium, as opposed to the much more stable cyclic isomer with two O-H...O hydrogen bonds, is attributed to the unique growth conditions that exist in helium droplets, at a temperature of 0.37 K. Theoretical calculations are also reported to aid in the interpretation of the experimental results. At long range the intermolecular interaction between the two monomers is dominated by the dipole-dipole interaction, which favors the formation of a polar dimer. By following the minimum-energy path, the calculations predict the formation of an acyclic dimer having one O-H...O and one C-H...O contact. This structure corresponds to a local minimum on the potential energy surface and differs significantly from the structure observed in the gas phase.