Polarized laser-induced photofragmentation studies of JKM-selected CH3I molecules: retention of orientation in optical and DC electric fields

The technique of linearly polarized laser-induced photofragmentation for the measurement of the degree of orientation of rotationally state-selected symmetric top molecules [Phys. Rev. Lett.59, 2951 (1987)] has been used to study the retention of molecular orientation in optical frequency AC and homogeneous DC electric fields. For CH₃I beams, state-selected by the electrostatic hexapole focuser in several specific low-J parent states, recoupling of the iodine nuclear spin with the molecular rotational angular momentum occurs rapidly in weak fields, leading to some loss of orientation, but the resulting degree of orientation (i.e., theM _F distribution) is retained in both DC and optical frequency electric fields. The direction of the orientation of the molecular axis can be inverted with 100% efficiency by changing the sign of the DC ‘orienting field’. The up/down asymmetry of the photofragment angular distribution can be observed with either parallel (vertical) or perpendicular (horizontal) laser polarization.     

Vector correlations in photodissociation of polarized polyatomic molecules beyond the axial recoil limit

We present the full quantum mechanical theory of the angular momentum distributions of photofragments produced in photolysis of oriented/aligned parent polyatomic molecules beyond the axial recoil limit. This paper generalizes the results of Underwood and Powis(28,29) to the case of non-axial recoil photodissociation of an arbitrary polyatomic molecule. The spherical tensor approach is used throughout this paper. We show that the recoil angular distribution of the angular momentum polarization of each of the photofragments can be presented in a universal spherical tensor form valid for photolysis in diatomic or polyatomic molecules, irrespective of the reaction mechanism. The angular distribution can be written as an expansion over the Wigner D-functions in terms of the set of the anisotropy-transforming coefficients c(K(i)q(i))(K) (k(d), K(0)) which contain all of the information about the photodissociation dynamics and can be either determined from experiment, or computed from quantum mechanical theory. An important new conservation rule is revealed through the analysis, namely that the component q(i) of the initial reagent polarization rank K(i) and the photofragment polarization rank K onto the photofragment recoil direction k is preserved in any photolysis reaction. Both laboratory and body frame expressions for the recoil angle dependence of the photofragment angular momentum polarization are presented. The parent molecule polarization is shown to lead to new terms in the obtained photofragment angular distributions compared with the isotropic case. In particular, the terms with |q(i)| > 2 can appear which are shown to manifest angular momentum helicity non-conservation in the reaction. The expressions for the coefficients c(K(i)q(i))(K) (k(d), K(0)) have been simplified using the quasiclassical approximation in the high-J limit which allows for introducing the dynamical functions and the rotation factors which describe the decreasing of the photofragment angular momentum orientation and alignment due to the rotation of the molecular axis during photodissociation. In this case, the resultant recoil angle dependence is also presented in a form where the anisotropy of the parent molecular ensemble is expressed in terms of the molecular axis distribution, rather than in terms of the molecular density matrix.     

Photofragment spectroscopy of molecular ions: design and performance of a new apparatus using coaxial beams

A new apparatus is described for the study of photofragment spectroscopy of molecular ions in a coaxial laser-ion beam configuration. Complementary to other developments in this field, the apparatus emphasizes the kinetic energy spectroscopy of the photofragments and is particularly designed to study direct photodissociation processes via repulsive potential curves. On the laser side, the experiment uses discrete lines in the visible and UV region (Ar⁺ laser, excimer laser). A detailed analysis of the experimental conditions is presented, in particular with respect to the attainable energy resolution in the photofragment spectra. The apparatus provides sufficiently intense ion beams (5–10 nA) with controlled energy resolution (ΔE = 100?500 meV) and angular collimation (ΔΘ = 5?10 mrad). The measured photofragment spectra of H ₂ ⁺ in the visible and UV region are in full accordance with the predictions of the design analysis.     

Approximate D4d symmetry of the cation site in hexahydrated trichlorides of rare earths and actinides

Many spectroscopic and magnetic properties of the monoclinic hexahydrated chlorides of rare earths have been shown to approximate those of a uniaxial crystal. We explain this approximate symmetry by the shape of the coordination polyhedron of the rare-earth ions which is approximately that of a square antiprism of D_4d symmetry, with an inverse eightfold axis. The directions of this axis and of the spectroscopically or magnetically determined axes agree well. The absence of M_J mixing for some levels, as observed in the Zeeman effect, can be explained by an eightfold axis. As a first approximation, we propose a study of the magnetic ions using a D_4d crystal field hamiltonian with the limited set of even parameters B₀², B₀⁴ and B₀⁶; polarization and intensity spectral data, Zeeman effect, electron paramagnetic resonance and susceptibility measurements can be interpreted in this symmetry. Such remarks also apply to actinide hexahydrated chlorides.     

An optically pumped,highly polarized cesium beam for the study of spin-dependent electron scattering

We have set up an atomic beam of cesium for the study of spin-dependent electron-cesium scattering. The beam is produced by an effusive oven with continuous recirculation of the condensed metal. The beam is optically pumped by circularly polarized light from two laser diodes tuned to the 6² S _1/2(F=3)→6² P _3/2(F′=4) and 6² S _1/2(F=4)→6² P _3/2(F′=5) transitions, respectively. Nearly all atoms are transferred into theF=4,m _F =+4 orm _F =?4 Zeeman level of the ground state, depending on the sense of circular polarization of the pumping light. The population distribution in the optically pumped beam is analyzed in terms of them _J =?1/2 andm _J =+1/2 components with a Stern-Gerlach magnet. We find the atomic polarization to be very close to unity at a density of 8×10⁸ atoms/cm³ in the scattering center. The polarization decreases slightly with increasing density of the cesium beam due to radiation trapping. A spin flipper serves as a means of polarization reversal, introducing no systematic errors in the spin asymmetry measurements. Lock-in technique is used to stabilize the atomic beam polarization by detecting fluorescence light signals.     

Photofragmentation of SnCl2 at 193 nm

Using photofragment spectroscopy the time of flight spectra and angular distributions for SnCl fragments, Sn and Cl atoms from the fragmentation of SnCl₂ have been measured at 193 nm. From the time, of flight it was found that dissociation takes place into SnCl (X ²Π) and Cl(²P) and that the SnCl fragment is highly vibrationally excited. To account for this effect we propose a mechanism, in which the upper potential surface has a saddle point at a greater Sn-Cl₂ distance than the ground state. The angular distribution exhibits an anisotropy parameter β = 0.21. A model was developed, based on a charge transfer from the p nonbonding orbitals on the Cl atoms to the p_x orbital on the Sn atom. The model predicts that A₁ → B₂ transitions dominate over A₁ → B₁ transitions and that A₁ → B₁ transitions are negligible in good agreement with the experimental observations.     

Photofragment angular momentum distribution beyond the axial recoil approximation: predissociation

We present the quantum mechanical expressions for the angular momentum distribution of the photofragments produced in slow predissociation. The paper is based on our recent theoretical treatment [J. Chem. Phys. 123, 034307 (2005)] of the recoil angle dependence of the photofragment multipole moments which explicitly treat the role of molecular axis rotation on the electronic angular momentum polarization of the fragments. The electronic wave function of the molecule was used in the adiabatic body frame representation. The rigorous expressions for the fragment state multipoles which have been explicitly derived from the scattering wave function formalism have been used for the case of slow predissociation where a molecule lives in the excited quasibound state much longer than a rotation period. Possible radial nonadiabatic interactions were taken into consideration. The optical excitation of a single rotational branch and the broadband incoherent excitation of all possible rotational branches have been analyzed in detail. The angular momentum polarization of the photofragments has been treated in the high-J limit. The polarization of the photofragment angular momenta predicted by the theory depends on photodissociation mechanism and can in many cases be significant.     

Zeeman effects in absorption and luminescence of the zero-phonon lines in Ru(bpy)3(PF6)2

Zeeman measurements made near 17815 cm⁻¹ at 4.2 and 1.4 K on a Ru(bpy)₃(PF₆)₂ single crystal, in both absorption and luminescence, are presented. Most significantly, no effect is seen with B||c, but substantial shifts are seen with B⊥c. These measurements are in direct conflict with the interpretation of a body of recent work purporting to determine the electronic structure of the Ru(bpy)²⁺₃ (RBY) moiety by a study of the temperature dependence of the luminescence of a number of concentrated salts of RBY. The anomalous dependence on the relative orientation of B and the detecting polarization vector E is qualitatively similar to measurements on dilute RBY systems. The nature of the electronic structure of the RBY luminophore below 10 K is discussed.     

Molecular photofragment orientation in the photodissociation of H2O2 at 193 nm and 248 nm

Angular momentum orientation has been observed in the OH(X(2)Π, v = 0) fragments generated by circularly polarized photodissociation of H(2)O(2) at 193 nm and 248 nm. The magnitude and sign of the orientation are strongly dependent on the OH(X) photofragment rotational state. In addition to conventional laser induced fluorescence methods, Zeeman quantum beat spectroscopy has also been used as a complementary tool to probe the angular momentum orientation parameters. The measured orientation at 193 nm is attributed solely to photodissociation via the ?(1)A state, even though at this wavelength H(2)O(2) is excited near equally to both the ?(1)A and B(1)B electronic states. This observation is confirmed by measurements of the photofragment orientation at 248 nm, where access to the ?(1)A state dominates. Several possible mechanisms are discussed to explain the observed photofragment orientation, and a simple physical model is developed, which includes the effects of the polarization of the parent molecular rotation upon absorption of circularly polarized light. Good agreement between the experimental and simulation results is obtained, lending support to the validity of the model. It is proposed that photofragment orientation arises mainly from the coupling of the parent rotational angular momentum with that induced during photofragmentation.     

Optical and ODMR investigation of the low lying excited states of neat α-oxalic acid dihydrate

The lowest excited singlet and triplet states of neat α-oxalic acid dihydrate have been investigated by optical, optical Zeeman, and zero-field optically detected magnetic resonance (ODMR) spectroscopy at T ? 4 K. The observed electronic transitions in absorption are assigned as ¹A_u ← ¹A_g (ν₀ = 34131 cm^?1) from its normal polarizatio These correspond to the expected lowest lying ^1,3nπ* excitations in trans-α-dicarbonyls. The ³A_u → ¹A_g phosphorescence is also observed. Monitoring the phosphorescence intensity, the fine structure splittings and principal axes' orientation and the kinetic parameters of the ³A_u s The fine structure constants are X = 2510.0, Y = ?1800.3, and Z = ?709.7 MHZ where the x axis is in-plane and parallel to the carbo The absolute signs of the constants have been established by optical Zeeman measurements. The τ_x zero-field spin state has the largest total phosphorescence rate, radiative rate, and populating rate. The τ_x activity in the 0 - 0 band is polarized mainly along the x axis. However, considerable normal polarization associated with an in-p     

Stark and zeeman effects on the lower electronic states of s-triazine

A detailed optical study of the lower electronic states of s-triazine in a pure crystal at 1.8°K is presented. Stark and Zeeman experiments on these states give no support to previous assignments. The experiments indicate that the lowest triplet state observed in s-triazine corresponds to either a polar A″₁ state or to an E″ state in which the orbital angular momentum is completely quenched. Optical measurements on a very weak triplet state absorption at 28930.7 cm^?1 in s-triazine are also reported. The premilinary assignment of this line as a crystal field induced Davydov component is shown not to be inconsistent with its Zeeman effect.     

Three-body dissociation dynamics of (SO2)3 studied through dissociative photodetachment of (SO2)3−

The dissociative photodetachment dynamics of (SO₂)₃⁻ were studied by photoelectron–photofragment coincidence spectroscopy at 258 nm. Correlation between the photoelectron and photofragment translational energies was observed as previously seen in the dimer system, implying the presence of a dimer core. The three-body dissociation dynamics of (SO₂)₃ after photodetachment are consistent with a dimer core solvated by a spectator SO₂ molecule with a broad distribution in initial geometry.     

Quasi-classical trajectory approach to the stereo-dynamics of the reaction F+HO→HF+O

Quasi-classical trajectory (QCT) calculations are employed for the reaction F + HO(0,0)→HF + O based on the adiabatic potential energy surface (PES) of the ground 3A″triplet state. The average rotational alignment factor P2(j′·k) as a function of collision energy and the four polarization dependent generalized differential cross sections have been calculated in the center-of-mass (CM) frame, separately. The distribution P(θr) of the angle between k and j′, the distribution P(θr) of dihedral angle denoting k-k′-j′ correlation, and the angular distribution P(θr, Φr) of product rotational vectors in the form of polar plots are calculated as well. The effect of Heavy-Light-Heavy (HLH) mass combination and atom F's relatively strong absorbability to charges on the alignment and the orientation of product molecule HF rotational angular momentum vectors j′ is revealed.     

Interpretation of the parameters of the EPR spectra of complexes containing ns 1 ions

The parameters of the EPR spectra of complexes containing paramagnetic ions with an unpaired ns electron (ns ¹ ions) were interpreted. The effect of the ligand spin-orbital coupling on the parameter of the Zeeman splitting was discussed. The effect of spin polarization on the parameters of hyperfine and ligand hyperfine couplings was considered. The reasons for the anomalous behavior of the EPR parameters were noted. The character of the covalent bonding was analyzed from the EPR spectra. The anomalous behavior of the parameters of ligand hyperfine couplings in tetragonal complexes with ns ¹ ions was discussed.     

Direct observation of the angular distribution of the chemiluminescence from Ba + N2O → BaO + N2

The angular distribution of the chemiluminescent reaction Ba + N₂O → BaO + N₂ has been investigated by photographing directly the chemiluminescence from this reaction in a crossed beam experiment. It was found that the lifetime of the reactively scattered chemiluminescent BaO molecules is sufficiently long (≈ 10^?s δ) to allow the observation of the angular distribution. From the dependence of this distribution on R and ? where R is the distance from the scattering center and ? the laboratory scattering angle, we conclude that under single collision conditions the chemiluminescence arises preferentially from highly excited vibrational-rotational levels of the A′¹ Π state of BaO.     

Kinetics of photo-induced dissociation of Na clusters deposited on Mica

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE _kin of the photodesorbed atoms at the surface temperatureT _S =300 K was found to beE _kin=18±5 meV, independent of laser irradiance (3 µJ/cm²...20 mJ/cm²) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E _kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT _S =30 K andT _S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos²-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.     

On the photoexcitation of the 2s→2p transition in light muonic atoms

It is known that the energy of the 2s→2p transition of light μ-atoms can be precisely measured by laser spectroscopy. Such measurements provide a good test of quantum electrodynamics predictions. Here we consider how the hyperfine muon-nucleus interaction effects on the photoexcitation of this transition and on the subsequent X-rays emission. Besides the obvious change of the transition energy the hyperfine interaction mixes the 2p 1/2 and 2p 3/2 orbits. This mixing is rather effective in μ-atoms of¹H,⁷Li,⁹Be,¹⁰B and¹¹B. Its taking into account changes the photoexcitation cross-section of electric dipole transitions between the hyperfine components of the 2s and 2p orbits and an angular X-rays distribution as well. These changes prove to be considerable for some transitions. For example, in μ²H and μ⁷Li the cross-section is decreased by factors of two and four respectively. In μ⁹Be it is increased by a factor of two. Moreover, in μ¹H the angular X-rays distribution becomes more anisotropic. Besides the above subject one more question is discussed here. The fact is that the laser experiments can give an information on the residual muon spin polarization at the 2s orbit and on the degree of alignment of the angular μ-atom momentum in this state. The polarization can be determined by measuring an angular correlation between the X-rays emission direction and the momentum of a hard μ-decay electron. The alignment degree can be found by measuring an anisotropy of the angular X-rays distribution.     

A role of the hyperfine interaction of the negative muon and an electron shell in formingP-odd correlations in the muonic ion μ20 Ne

The weak interaction of neutral currents of the negative muon and nucleons results in arisingP-odd correlations in the radiative 2s→ 1s muon-ion transition. At the present paper we consider how the hyperfine muon-electron interaction influences forming these correlations in the muonic ion μ²⁰ Ne with oneK-electron. It is shown that the general form of expressions describing aP-odd asymmetry of an angular distribution of the quanta and their circular polarization does not depend on electron configurations. On the other hand such dependence takes place for expressions describing an angular correlation between the momenta of the quantum and the hard electron emitted in the μ-decay process at the 1s-orbit. Therefore we think that measuring theP-odd asymmetry of the angular distribution is the preferable experiment for studying the neutral currents interaction. The spin polarization of the muon at the 2s-orbit can be obtained by measuring the circular polarization of the quanta in the same experiment.