The effect of nuclear spin on the polarisation ratio of resonance fluorescence

The nuclear quadrupole interaction causes the rotational angular momentum of a molecule with nuclear spin to precess about the total angular momentum whose projection along a space fixed axis is conserved. This effects the degree of orientation in the excited state of molecules optically pumped with circularly polarised light and reduces the polarisation ratio of fluorescence from molecules with large nuclear spin. Calculations are presented for the circular polarisation ratio of the forward scattered fluorescence when the exciting radiation is sufficiently broad-banded that it excites the full manifold of quadrupole split components of a given J″ → J′ transition. Results are also presented for the variation of the polarisation ratio and intensity as a narrow excitation source is tuned across the absorption.     

Multiphoton ionisation spectroscopy of H2S: a reinvestigation of the 1B1-1A1 band at 139.1 nm

The 3 + 1 multiphoton ionisation (MPI) spectrum of the ¹B₁-¹A₁ transition in H₂S at 139.1 nm has been recorded in both linearly and circularly polarised light. The rotational structure shows marked differences from that of the one-photon absorption spectrum. Properties of the excited state revealed through analysis of this structure include confirmation of its ¹B₁ character, refined values for its A, B and C rotational constants and the operation of an energy-dependent predissociation mechanism. It is shown that the third-rank tensor component of the transition operator dominates over the first-rank component in this MPI band. The orbital nature of this ¹B₁ excited state is considered.     

Spectral measurement of the caesium D2 line with a tunable heterodyne interferometer

The optical properties of a caesium atomic beam driven on a resonant hyperfine transition in the D₂ line were studied as a function of the probe laser frequency. Using a third off-resonant laser system, a heterodyne interferometer allowed simultaneous absorption and phase shift measurements of either the probe or the coupling laser. The signal features of the probe and coupling laser transmitted intensities showed strong differences in the vicinity of the hyperfine transitions excited by the probe laser. Regular absorption signals and electromagnetically induced transparency were found in either transmitted intensities. Furthermore, light induced birefringence of the probe laser was measured.     

Selection rules for collisional energy transfer in homonuclear diatomics. rotationally inelastic collisions

We have made a precise study of the circular polarisation of rotationally resolved features of laser-excited iodine. The J′ = 19, ν′ = 16 level of ³II⁺_ou was excited using circularly polarised dye-laser fluorescence and a quantitative data on polarisation features representing inelastic transfer of ΔJ′ = 30 was recorded. The experimental circular polarisation ratios were compared to those predicted by two totally conserving models, ΔM = 0 and Δθ = 0. The agreement between experimental points and the predictions based on the former lead to the formation of a new selection due on rotationally inelastic transfer namely, ΔM = 0.     

Amplification of electroweak left-right asymmetry in atoms by stimulated emission

This work describes quantitatively the amplification of the electroweak left-right asymmetry, a remarkable and attractive feature of the experiment in progress on the 6S _1/2 → 7S _1/2 cesium transition using detection by stimulated emission on the 7S _1/2 → 6P _3/2 transition. The process relies on the optical anisotropy of the atomic medium resulting from the 7S _1/2 alignment created by excitation with linearly polarized light. The crucial parameter (α_⊥?α_∥)/α_∥ involves the amplification coefficients for the probe field oriented in a direction either parallel or perpendicular to the alignment axis. Explicit computations are done by a semiclassical approach (classical for the field, quantum mechanical for the atomic states). The larger the optical anisotropy and the optical density, the stronger the asymmetry amplification; among all hyperfine components of the 7S _1/2 → 6P _3/2 transition, the ΔF=0 ones and more particularly the 4 → 4 offer the largest anisotropy. It is also predicted that saturation of the probe transition by the optical field should provide manifestation of the effect at lower optical densities and notwithstanding at larger fluxes of transmitted photons. Indirect production of 6P _3/2 atoms by the excitation pulse does not reduce the left-right asymmetry, neither its amplification which then appears at lower effective optical densities.     

Near resonant charge transfer in Na(4D)+K+→Na+ + K*: optical pumping of the Na(4D) state and energy dependence of rank 4 alignment

Rank 4 alignment has been observed in a quasi one electron system. Relative charge transfer cross sections of the Na 4dσ, 4dπ and 4dδ sublevels for the K⁺+Na(4D)→K*+Na⁺ system have been measured. A strong energy dependence is observed. The results at energies less than 1 keV may be attributed to rotational coupling of the Na 4dπ state to the K 4fδ state. The Na atom is excited in a two-step process, the first step being excitation to the 3² P _3/2 level with a two-mode laser to pump from both hyperfine levels of the ground state simultaneously. This two-mode laser is described in detail. The optical pumping problem is solved using rate equations. The general formula for describing the scattering intensity for cylindrical symmetry, in terms of multipole moments, for atoms excited by two linearly polarised lasers is derived and used to evaluate the experimental results.     

Differential sub-Doppler,one-photon,optical absorption spectroscopy of gases,and vapors with lasers

We consider in this work a high resolution, Doppler-free, one-photon, optical absorption differential spectroscopy of a mixture of two different mutually interacting gases or vapors in order to display the effects of this interaction directly in the optical spectra. Those interactions are restricted to binary collisions and they are treated in the so-called impact approximation (hard collisions). Usually, the high resolution, Doppler-free optical spectra are observed when two counterpropagating laser beams pass through the gas cell. One beam is strong enough to saturate the optical transition to several different degrees. The other beam is a weak probe absorbed by the saturated atomic or molecular pair of energy levels (a two-energy level system with angular frequency ω₀). The laser beams are supposed to have different angular frequencies and to be linearly polarized. In order to achieve the Doppler-free differential optical absorption one-photon spectra, a convenient geometrical experimental set up is proposed.     

Measurement of kinetic processes in photoexcited stilbene solutions

The kinetics of photophysical processes has been measured in stilbene solutions in the temperature range between ?40°C and 20°C. The population of the S₁ level excited by two-photon absorption (TPA) and of secondary populated levels has been investigated using a probe beam method. It was found an energy barrier ΔE = 5 × 10² cm^?1 of the thermically activated transition S₁ → 1′. The rate parameter of this transition is determined to be k^?1_11′, ≈ 2 ps at very high temperatures.     

Unusual photo-induced behaviour in a side chain liquid crystalline azo-polyester

An unusual behaviour has been observed in the photo-induced response of an azobenzene side chain liquid crystalline polyester (P6d4). Room temperature irradiation with linearly polarised 488 nm light does not induce any birefringence (Δn) in films of this polymer that have been quenched from the isotropic state. However, using the same irradiation conditions Δn is induced in quenched films that have been kept in darkness for a few minutes. Besides, no photo-induced Δn is observed in films irradiated with 488 nm light that have been previously irradiated with UV light. In this case, Δn can be recorded if the UV irradiated films have been kept in darkness for several hours. In another set of experiments performed with the P6d4 polymer, irradiation with high intensity linearly polarised 488 nm light induces an initial increase of Δn and then it goes back to zero. Subsequent irradiation with linearly polarised 633 nm light induces an orientation of the azobenzene chromophores perpendicular to the polarisation of the 488 nm light and independent of the 633 nm light polarisation direction. These results are very different from those obtained in other side chain polyesters which only differ from P6d4 in the end substituent. The anomalous behaviour of P6d4 is discussed in terms of a supercooling effect of the isotropic phase, the trans to cis back isomerization, the trans aggregation kinetics and a memory effect associated with the main chain orientation.     

AB initio calculation of vertical transition energies of the C2H2 molecule

Various ab initio SCF and Cl treatments are reported for the acetylene molecule in ground and excited states at its fixed linear equilibrium geometry. The results speak in favor of the former analysis of the vacuum UV absorption spectra of C₂H₂ given by Wilkinson according to which above the X?C state an allowed transition to a linear upper state (X?D) and two allowed transitions with broad bands (X?E and X?F) to non-linear upper states are present. The calculated order and spacing of the excited states does not support the new analysis given by Jungen according to which only one allowed transition (X?F) and a forbidden one to a linear upper state (X?D) should appear between 1250 and 1360 A.     

On the scattering of a light pulse on a single atom perturbed by collisions with inert gas atoms

Scattering of a laser pulse on a single atom perturbed by collisions with inert gas atoms is described in terms of suitably defined cross-sections. The relations of the energy balance are explored to show the connection between attenuation and total cross-section for scattering. The different scattering channels, corresponding to the polarisation, spectral decomposition and coherence properties of the light are analysed in terms of the cross-sections. As an example we consider the scattering of a light pulse on an alkali metal atom with a frequency close to the atomic transition frequency from the ground stateS _1/2 to the first excited stateP _1/2.     

Circular emission from molecular iodine

The resonance emission from molecular iodine excited by circularly polarised light is circularly polarised. We report the circular emission spectrum of iodine excited by the 5145 A line of Ar⁺. In zero field the prominent rotational triplet and the high energy P,R doublet are strongly circularly polarisation ratios close to theoretical.     

Identify fine microstructure of multifarious iron oxides via O K-edge EELS spectra

Relying on the electron energy loss spectrum (EELS) of metallic elements to obtain microstructure analysis is an investigation method of the reaction mechanisms of transition metal oxides (TMOs) in catalysis, energy storage and conversion. However, the low signal from K shell owing to insufficient electron beam energy, and the complicated electronic structure in L shell of the metal element restrict the analysis of the coordination environment of the TMOs. Herein, density functional theory (DFT) calculation, Fourier transform (FT) and wavelet transform (WT) were employed to probe the relationship between the four individual peaks in O K-edge spectra of iron oxides and the microstructure information (chemical bonds and atomic coordination). The findings show that the peak amplitude ration is in a linear correlation with the valence state of Fe element, and that the coordination number obtained by radial distribution function (RDF) is favorably linearly correlative with that from the standard coordination structure model. As a result, the quantitative analysis on the change of valence state and atomic coordination in microstructure can be realized by EELS O K-edge spectra. This study establishes EELS O K-edge spectrum as a promising pathway to quantitatively analyze the valence state and atomic coordination information of TMOs, and offers an effective method to conduct microstructure analysis via the EELS spectra of the non-metal element.     

Structured weak absorption spectrum of p-terphenyl microcrystals dispersed in methylcyclohexane glass matrix at 77K

P-terphenyl microcrystals dispersed in a methylcyclohexane glass matrix at 77 K showed a sharp structured weak absorption system at lower energy than the well known strong conjugation band. We assign it to the short-axis polarized transition, ¹B_3u←¹A_g. It is concluded that the lowest excited singlet state is a long-lived state, ¹B_3u, in a planar conformation, whereas it is a short-lived state, ¹A_u, in a twisted conformation.     

Linear and nonlinear optical studies of CdS1−x Se x quantum dots

In this contribution we present and discuss our measurements on CdS_1?x Se _x quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum.     

Spectral measurement of the caesium D(2) line with a tunable heterodyne interferometer

The optical properties of a caesium atomic beam driven on a resonant hyperfine transition in the D(2) line were studied as a function of the probe laser frequency. Using a third off-resonant laser system, a heterodyne interferometer allowed simultaneous absorption and phase shift measurements of either the probe or the coupling laser. The signal features of the probe and coupling laser transmitted intensities showed strong differences in the vicinity of the hyperfine transitions excited by the probe laser. Regular absorption signals and electromagnetically induced transparency were found in either transmitted intensities. Furthermore, light induced birefringence of the probe laser was measured.     

Absorption spectra of crystalline thiophthene

The near ultraviolet absorption spectra of crystalline thiophthene have been measured in polarised light at temperatures down to 4–6 K. The spectra show that only the Davydov component located at higher energies remains broad at low temperature. The nature of this broadening is discussed in terms of downward inter-band phonon scattering from the upper optical level by a librational phonon. A comparison of experimental data and theoretical calculations of energy levels and polarisation ratios based on the dipole—dipole approximation has been carried out, and the direction of polarisation of the π* ← π electronic transition which lies in the thiophthene molecular plane has been defined.     

Measurement of excited state population decay time in NH3 by optical adiabatic rapid passage

A strong Q-switched N₂O laser capable of 60 MHz frequency sweep is used to induce population inversion by optical adiabatic rapid passage on the ν₂[asQ(8,7)] transition of ¹⁴NH₃. The fast inversion and the subsequent return to equilibrium are detected by a cw weak counterpropagating N₂O laser, and are used to measure the pressure dependence of excited state population relaxation time T₁ for the infrared transition of interest. this decay is found to be 3.6 times slower than the correpsonding decay in the ground state, an effect which may be explained by the larger inversion energy separation in the excited state of NH₃.     

Magneto-chiral Nonlinear Optical Effect with Large Anisotropic Response in Two-Dimensional Halide Perovskite

The chiral organic–inorganic halide perovskites (OIHPs) are vital candidates for superior nonlinear optical (NLO) effects associated with circularly polarized (CP) light. NLO in chiral materials often couples with magnetic dipole (MD) transition, as well as the conventional electric dipole (ED) transition. However, the importance of MD transition in NLO process of chiral OIHPs has not yet been well recognized. Here, the circular polarized probe analysis of second harmonic generation circular dichroism (SHG-CD) provides the direct evidence that the contribution of MD leads to a large anisotropic response to CP lights in chiral OIHPs, (R-/S-MBACl)₂PbI₄. The thin films exhibit great sensitivity to CP lights over a wide wavelength range, and the g-value reaches up to 1.57 at the wavelength where the contribution of MD is maximized. Furthermore, it is also effective as CP light generator, outputting CP-SHG with maximum g-factor of 1.76 upon the stimulation of linearly polarized light. This study deepens the understanding of relation between chirality and magneto-optical effect, and such an efficient discrimination and generation of CP light signal is highly applicable for chirality-based sensor and optical communication devices.     

The calculation of London coefficients by the variation-perturbation method

The London coefficients for the dispersion interaction between LiLi, BeBe and LiBe are calculated by the variation-perturbation method using the whole atomic hamiltonian as H₀ and the Hartree-Fock approximation for the upper turbed wavefunction ?₀. A single excited valence configuration with optimized orbital exponents gives accurate results for Li, whereas comparable accuracy for Be is obtained when part of the valence correlation energy in the ground as well as in the excited state is accounted for through a limited configuration interaction.