Manipulating the transmission of vector beam with spatially polarized atomic ensemble.

Vector beams (VBs) with potential applications are successfully utilized in many fields as light sources with a spatially-varying polarization profile in recent years. Here, we study the transmission of a VB by manipulating atomic polarization via the optical pumping effect. By using hybridly and radially polarized beams as pump and probe beams in a counter-propagating configuration, we observe a four-petal pattern intensity distribution of probe beam, and the four-petal pattern rotates with the polarization state orientation of the pump beam. The results show a polarization dependent absorption in the atomic media. We experimentally demonstrate the absorption characteristics under different polarization combinations of pump and probe beams. The Jones matrix method is used to explain this phenomenon and the simulations are consistent with the experimental observation. Our results may provide a sound foundation for applications in optical manipulation and quantum information in atomic ensembles.

Binding energies of the π-stacked anisole dimer: new molecular beam-laser spectroscopy experiments and CCSD(T) calculations.

Among noncovalent interactions, π-π stacking is a very important binding motif governed mainly by London dispersion. Despite its importance, for instance, for the structure of bio-macromolecules, the direct experimental measurement of binding energies in π-π stacked complexes has been elusive for a long time. Only recently, an experimental value for the binding energy of the anisole dimer was presented, determined by velocity mapping ion imaging in a two-photon resonant ionisation molecular beam experiment. However, in that paper, a discrepancy was already noted between the obtained experimental value and a theoretical estimate. Here, we present an accurate recalculation of the binding energy based on the combination of the CCSD(T)/CBS interaction energy and a DFT-D3 vibrational analysis. This proves unambiguously that the previously reported experimental value is too high and a new series of measurements with a different, more sensitive apparatus was performed. The new experimental value of 1800±100 cm(-1) (5.15±0.29 kcal mol(-1)) is close to the present theoretical prediction of 5.04±0.40 kcal mol(-1). Additional calculations of the properties of the cationic and excited states involved in the photodissociation of the dimer were used to identify and rationalise the difficulties encountered in the experimental work.

Structure and energetics of the anisole-Ar(n) (n = 1, 2, 3) complexes: high-resolution resonant two-photon and threshold ionization experiments, and quantum chemical calculations.

We present a concerted experimental and theoretical study of the anisole···Arn complexes with n = 1-3. Experimentally, anisole was seeded into a pulsed supersonic argon jet producing a molecular beam. Resonant two-photon, two-colour ionisation (R2PI) spectra of anisole···Arn complexes with n = 1-3 were obtained. Also, the photodissociation of the (1 : 1) cluster was probed synchronously by - Zero Electron Kinetic Energy Photoelectron Spectroscopy (ZEKE) - and - Mass Resolved Threshold Ionization (MATI) - measuring electrons and ions obtained from pulsed field ionization of high-n Rydberg states upon two-colour laser excitation. The experimental results are compared to quantum chemical calculations at the DFT-D3 (B-LYP/def2-QZVP level with Grimme's D3 dispersion correction) level. Structure and energetics due to microsolvation effects by the direct interaction of the argon atoms with the π-system were evaluated. The experimental binding energy of the 1 : 1 cluster is finally compared to computational results; in the S0 ground state the theoretical value based on the "gold standard" CCSD(T)/CBS calculations lies within the error bars of the observed value. In the excited state the agreement between theory and experiment is not so spectacular but relative values of observed dissociation energies (D0) in the ground and excited states and of calculated ones agree well.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol-Ar cluster near the ionization threshold.

The structure of the phenol-argon cluster (PhOH-Ar) in high-n Rydberg states is investigated by the newly developed technique of mass analyzed threshold ionization detected infrared (MATI-IR) spectroscopy. This method selectively measures IR spectra of molecular clusters in very high-n Rydberg states (n > 100) utilized in zero kinetic energy (ZEKE) photoelectron and MATI spectroscopy, whose ionic cores are essentially the same as the corresponding bare cation. The MATI-IR spectrum exhibits only the free OH stretching vibration (ν) when the π-bound cluster of the neutral ground electronic state (S0) is resonantly excited via the S1 origin to Rydberg states converging to its adiabatic ionization energy level, IE0(π). When Rydberg states converging to vibrationally excited levels of the local π-bound minimum are prepared, in addition to ν also the hydrogen-bonded OH stretching vibration (ν) of the H-bonded global minimum is observed in the MATI-IR spectra, even for vibrational excitation of only 14 cm(-1) above IE0(π). These results show that the π→ H site switching reaction of the Ar ligand from the aromatic ring to the OH group proceeds only from vibrationally excited states in the π-bound cation core with a small barrier of less than 14 cm(-1) from IE0(π). On the other hand, directly photoionized PhOH(+)-Ar shows both ν and ν in the IR spectra, even when it is just ionized to IE0(π). This result implies that the ionization-induced π→ H site switching occurs without excess energy in the H-bound or π-bound cations, in contrast to very high-n Rydberg states converging to levels of the π-bound cation. The different efficiencies of the site switching for the Rydberg ion core and the bare ion and the mechanism for the π→ H site switching are interpreted by direct ionization from the π-bound to the H-bound structures in addition to the conventional vertical ionization and transitions to high-n Rydberg states.

Ionization-induced π → H site-switching in phenol-CH4 complexes studied using IR dip spectroscopy.

IR spectra of phenol-CH4 complexes generated in a supersonic expansion were measured before and after photoionization. The IR spectrum before ionization shows the free OH stretching vibration (ν(OH)) and the structure of neutral phenol-CH4 in the electronic ground state (S0) is assigned to a π-bound geometry, in which the CH4 ligand is located above the phenol ring. The IR spectrum after ionization to the cationic ground state (D0) exhibits a red shifted ν(OH) band assigned to a hydrogen-bonded cationic structure, in which the CH4 ligand binds to the phenolic OH group. In contrast to phenol-Ar/Kr, the observed ionization-induced π → H migration has unity yield for CH4. This difference is attributed to intracluster vibrational energy redistribution processes.

Spectral shifts and structures of phenol...Ar(n) clusters.

A laser spectroscopic investigation of phenol...Ar(n) (n = 1-6) clusters in the first electronically excited state (S(1)) and the cationic ground state (D(0)) is reported. Resonance enhanced two-photon ionisation (R2PI) spectra have been recorded for the investigation of the S(1) state. The origins of S(1)← S(0) (S(1)0(0)) transition of phenol...Ar(n) (n = 1, 2,4-6) are all red shifted compared to the S(1)0(0) state of the monomer by 33 cm(-1), 67 cm(-1), 10 cm(-1), 20 cm(-1), 44 cm(-1), respectively. However, the origin of the phenolAr(3) cluster is blue shifted by 25 cm(-1). For the investigation of the ionic ground state photoionization efficiency (PIE) and mass-analyzed-threshold ionization (MATI) spectroscopy have been applied. The spectra of phenol...Ar(3) and phenol...Ar(4) yield values for the ionization energy (IE) of 68,077 ± 15 cm(-1) and 67,948 ± 15 cm(-1). With the combination of theoretical methods and R2PI, PIE and MATI spectroscopy, the major species present have been positively identified.

Fragmentation energetics of the phenol(+)···Ar(3) cation cluster.

The various dissociation thresholds of phenol(+)···Ar(3) complexes for the consecutive loss of all three Ar ligands were measured in a molecular beam using resonant photoionization efficiency and mass analyzed threshold ionization spectroscopy via excitation of the first excited singlet state (S(1)). The adiabatic ionization energy is derived as 68077 ± 15 cm(-1). The analysis of the dissociation thresholds demonstrate that all three Ar ligands in the neutral phenol···Ar(3) tetramer are attached to the aromatic ring via π-bonding, denoted phenol···Ar(3)(3π). The value of the dissociation threshold for the loss of one Ar ligand from phenol(+)···Ar(3)(3π), ∼190 cm(-1), is significantly lower than the binding energy measured for the π-bonded Ar ligand in the phenol(+)···Ar(π) dimer, D(0) = 535 ± 3 cm(-1). This difference is rationalized by an ionization-induced π → H isomerization process occurring prior to dissociation, that is, one Ar atom in phenol(+)···Ar(3)(3π) moves to the OH binding site, leading to a structure with one H-bonded and 2 π-bonded ligands, denoted phenol(+)···Ar(3)(H/2π). The dissociation thresholds for the loss of two and three Ar atoms are also reported as 860 and 1730 cm(-1). From these values, the binding energy of the H-bound Ar atom can be estimated as 870 cm(-1).

Rotational analysis for the Doppler-free photoelectron spectrum of water using the spectator model.

A photoelectron spectrum of H(2)O has been recorded at a resolution of 2 meV under Doppler-free conditions. Complex rotational structures appear in the individual vibrational states of the electronic X̃(+ 2)B(1) and Ã(+ 2)A(2) states in H(2)O(+). The rotational structures are analyzed and well reproduced using a spectator orbital model developed for rotationally resolved photoelectron spectroscopy.

Dissociation energetics of the phenol(+)⋯Ar(2) cluster ion: The role of π→H isomerization.

The dissociation energetics in the phenol(+)⋯Ar(2)(2π) cluster ion have been investigated using photoionization efficiency and mass analyzed threshold ionization spectroscopy. The appearance energies for the loss of one and two Ar atoms are determined as ∼210 and ∼1115 cm(-1), respectively. The difference between the appearance energy for the first Ar ligand in phenol(+)⋯Ar(2)(2π) and the dissociation energy of the phenol(+)⋯Ar(π) dimer (535cm(-1)) is explained by the isomerization of one π-bound Ar ligand to the OH binding site (H-bond) upon ionization. The energy difference between phenol(+)⋯Ar(2)(2π) and phenol(+)⋯Ar(2)(H/π) could also be estimated to be around 325cm(-1), which corresponds roughly to the difference of the binding energy of a π-bound and H-bound Ar ligands. The binding energy of the H-bound Ar atom in phenol(+)⋯Ar(2)(H/π) is derived to be ∼905cm(-1).

The structure of phenol-Ar(n) (n=1,2) clusters in their S(0) and S(1) states.

The structures of the van der Waals bonded complexes of phenol with one and two argon atoms have been determined using rotationally resolved electronic spectroscopy of the S(1)<--S(0) transition. The experimentally determined structural parameters were compared to the results of quantum chemical calculations that are capable of properly describing dispersive interactions in the clusters. It was found that both complexes have pi-bound configurations, with the phenol-Ar(2) complex adopting a symmetric (1mid R:1) structure. The distances of the argon atoms to the aromatic plane in the electronic ground state of the n=1 and n=2 clusters are 353 and 355 pm, respectively. Resonance-enhanced multiphoton ionization spectroscopy was used to measure intermolecular vibrational frequencies in the S(1) state and Franck-Condon simulations were performed to confirm the structure of the phenol-Ar(2) cluster. These were found to be in excellent agreement with the (1mid R:1) configuration.

Effect of noncovalent interactions on the n-butylbenzene...Ar cluster studied by mass analyzed threshold ionization spectroscopy and ab initio computations.

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB...Ar clusters, and red shifts for anti-BB...Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I...Ar and BB-V...Ar were determined as 70052 and 69845 +/- 5 cm (-1), respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I...Ar and BB-V...Ar were obtained as 650 and 558 cm (-1), respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB...Ar in the excited neutral S 1 and the D 0 cationic ground states.

Effect of noncovalent interactions on conformers of the n-butylbenzene monomer studied by mass analyzed threshold ionization spectroscopy and basis-set convergent ab initio computations.

Two conformational isomers of the aromatic hydrocarbon n-butylbenzene have been studied using two-color MATI (mass analyzed threshold ionization) spectroscopy to explore the effect of conformation on ionization dynamics. Cationic states of g auche-conformer III and anti- conformers IV were selectively produced by two-color excitation via the respective S 1 origins. Adiabatic ionization potentials of the gauche- and anti-conformations were determined to be 70146 and 69872 +/- 5 cm (-1) respectively. Spectral features and vibrational modes are interpreted with the aid of MP2/cc-pVDZ ab initio calculations, and ionization-induced changes in the molecular conformations are discussed. Complete basis set (CBS) ab initio studies at MP2 level reveal reliable energetics for all four n-butylbenzene conformers observed in earlier two-color REMPI (resonance enhanced multiphoton ionization) spectra. For the S 0 state, the energies of conformer III, IV and V are above conformer I by 130, 289, 73 cm (-1), respectively. Furthermore, the combination of the CBS calculations with the measured REMPI, MATI spectra allowed the determination of the energetics of all four conformers in the S 1 and D 0 states.

A novel experimental system of high stability and lifetime for the laser-desorption of biomolecules.

A novel laser desorption system, with improved signal stability and extraordinary long lifetime, is presented for the study of jet-cooled biomolecules in the gas phase using vibrationally resolved photoionization spectroscopy. As a test substance tryptophane is used to characterize this desorption source. A usable lifetime of above 1 month (for a laser desorption repetition rate of 20 Hz) has been observed by optimizing the pellets (graphite/tryptophane, 3 mm diameter and 6 mm length) from which the substance is laser-desorbed. Additionally, the stability and signal-to-noise ratio has been improved by averaging the signal over the entire sample pellet by synchronizing the data acquisition with the rotation of the sample rod. The results demonstrate how a combination of the above helps to produce stable and conclusive spectra of tryptophane using one-color and two-color resonant two-photon ionization studies.

State of the art theoretical study and comparison to experiment for the phenol...argon complex.

The phenol...argon complex was studied by means of various high level ab initio quantum mechanics methods and high resolution threshold ionization spectroscopy. The structure and stabilization energy of different conformers were determined. Stabilization energy of van der Waals bonded and H-bonded PhOH...Ar complex determined at CCSD(T) complete basis set (CBS) level for CP-RI-MP2/cc-pVTZ/Ar aug-cc-pVTZ geometries amount to 434 and 285 cm(-1). The CCSD(T)/CBS were constructed either as a sum of MP2/CBS interaction energy and CCSD(T) correction term [difference between CCSD(T) and MP2 correlation energies determined with medium basis set] or directly from CCSD(T)/aug-cc-pVDZ and aug-cc-pVTZ energies. Both schemes provide very similar values. Harmonic vibrational analysis revealed that the H-bonded structure does not represent energy minimum but first order transition structure. The respective imaginary vibrational mode (16 cm(-1)) connects two possible argon locations -- above and below the phenol aromatic ring. Including the DeltaZPVE, we obtained stabilization enthalpy at 0 K of 389 cm(-1). This value is marginally higher (25-35 cm(-1), 0.07-0.10 kcal/mol) than the experimental value. The determination of DeltaZPVE constitutes the most significant error and possible improvements should come from more accurate evaluation of the (nonharmonic) vibrational frequencies.

Competition between stacking and hydrogen bonding: theoretical study of the phenol...Ar cation and neutral complex and comparison to experiment.

Experimental results obtained previously for vdW-bonded and H-bonded phenol...argon (PhOH...Ar) complexes in their S(0) and D(0) states are combined with ab initio quantum-chemical theoretical results. Such a combination allows us to present a "complete" description of the geometry, relative energies, interaction energies and enthalpies of PhOH...Ar complexes. Based on a minimum-energy-path study, the transition structures and barrier heights related to transitions between stable conformers are also presented. For the presented structures, the agreement between the theoretical interaction enthalpy at 0 K with experimentally obtained values is very good. On the other hand, for numerical harmonic-frequency calculations we find a very poor performance for the neutral PhOH...Ar complex and complete failure for the studied complexes in their cationic form.

Hole-burning spectra of phenol-Arn (n = 1, 2) clusters: resolution of the isomer issue.

The hole-burning (HB) spectra of phenol-Arn (PhOH-Arn) clusters with n = 1 and 2 have been measured in a molecular beam to clarify the possible existence of isomers. Two species were identified to give rise to signals in the S1-S0 spectrum recorded for the n = 1 cluster; however, one of the species was found to originate from dissociation of an n = 2 cluster. Similarly, three species were observed in the spectrum of the n = 2 cluster, and two of them were assigned to n = 3 and larger clusters. The spectral contamination from larger size clusters was quantitatively explained by the dissociation after photoexcitation. The analysis of the spectra demonstrates that only a single isomer exists in the molecular beam for both the n = 1 and the n = 2 clusters. In addition to two previously detected intermolecular modes, a third low-frequency mode, assigned to an intermolecular bending vibration, is observed for the first time in the HB spectrum of the n = 2 cluster. The assignments of the intermolecular vibrations were confirmed by ab initio MO calculations. The observation of the third intermolecular vibration suggests that the geometry of the n = 2 cluster has Cs or lower symmetry.