Two-photon absorption and energy band structure of orthorhombic Hg2Cl2 crystals

Dependence of two-photon absorption (TPA) rate on the state of polarization of a laser beam is investigated in the low-temperature orthorhombic modification of Hg₂Cl₂ crystals. Theoretical calculations of the dependence of TPA rate on the direction of polarization vector of the beam are performed for centrosymmetric points, Y, Z, T, R andS of the Brillouin zone. The domain structure of real crystals is taken into consideration and it is shown that periodicity of the polarization dependence may indicate whether TPA is due to transitions either at, Y, Z, T orR, S points. The polarization dependence of TPA cannot, however, distinguish between points inside these two groups. Comparison of theory with a low-temperature (T 8·5 K) experimental curve of polarization dependence is discussed. It is shown that the experiment can be explained in main features by a model of noninteracting oriented linear dipoles. Further, on the basis of TPA measurements, a simple energy band structure of Hg₂Cl₂ is proposed regarding Hg₂Cl₂ crystal as a linear chain of molecules.The authors express thanks to Dr. . Barta for supplying the Hg₂Cl₂ crystals, Dr. Z. Bryknar for critical comments and Dr. B. Velický for stimulating discussion. We thank also Dr. V. Kohlová for assistance during measurement.     

Two-photon polarization spectroscopy of LiF crystals with F 2 laser color centers

The paper reports on a measurement and calculation of the dependence of LiF: F ₂ luminescence polarization and intensity on the polarization of high-power laser radiation (λ_ex=1064 nm) producing two-photon excitation of F ₂ centers. It is shown that the two-photon transition results in the excitation of a previously unknown electronic level of F ₂ centers of the A _1g symmetry.     

Band structure and optical transitions in the Hg3Se2Cl2 crystals

First-principles calculations of the band structure for Hg₃Se₂Cl₂ crystal were performed by means of density functional theory (DFT). The exchange and correlation potential was described within a framework of the local density approximation based on exchange-correlation energy optimization to calculate the total energy. DOS/PDOS and valence charge distribution were studied in details. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions. For device applications based on Hg₃Se₂Cl₂ crystal, understanding these fundamental issues is highly important and essential. It should be noted that optoelectronic applications of Hg₃Se₂Cl₂ are caused by coexistence of the large polarized Hg cations and a huge contribution of an harmonic phonon subsystem caused by anions.     

Surface analysis of Hg2Cl2

A surface composition of (110) face of Hg₂Cl₂ in high vacuum is studied for the room temperature. The impact of primary electrons with energy of 2·5 keV causes a decomposition of the surface layer which is described in detail.The single crystals of Hg₂Cl₂ used in this work were prepared by Dr. Barta in Institute of Solid State Physics of Czechosl. Acad. Sci., Prague.     

Polar optic phonons in Hg2Cl2 and Hg2Br2

Far infrared (30–430 cm^?1) reflectivity measurements of Hg₂Cl₂ and Hg₂Br₂ single crystals have been performed in polarized light. The spectra, which are in agreement with group-theoretical predictions, were analyzed by the oscillator fitting procedure and Kramers-Kronig method. The results are compared with the existing data from other measurements and the large anisotropy of polar modes is briefly discussed. The polarization vectors of all long-wavelength symmetry modes were determined group-theoretically.     

Luminescence of defect centres in Hg2Cl2

Abstract

Luminescence in Hg₂Cl₂, crystals excited with UV light is investigated in the spectral region 0.8–2.25 eV. Measurements are performed on as-grown samples and on samples previously exposed to UV light at RT. Six emission bands are found which depend on the concentration of the photochemical entities produced by irradiation of the crystals at RT. It is concluded that infra-red (IR) luminescence of Hg₂Cl₂ originates from crystal defects. The observed emission bands are tentatively attributed to the emission of (HgCI_xBr_3?x)^? and (HgCl_xBr_4?x)^2? complexes formed with residual Br impurities. Centres responsible for IR Hg₂Cl₂ emissions are excited: (i) via excitons of Hg₂Cl₂, (ii) via excited states of isolated Hg₂Br₂ molecules, and (iii) resonantly through the excitation bands of defect centres.     

Clusters in the model ferroelastic Hg2Cl2

The critical fluctuations induced by a ferroelastic phase transition and corresponding to the soft TA mode at the X point at Brillouin zone boundary have been studied in the model crystal Hg₂Cl₂ within a broad temperature and wave-vector range. Anisotropic diffuse x-ray scattering maxima associated with order-parameter fluctuations and nucleation of low-temperature orthorhombic clusters in the high-temperature tetragonal matrix have been found to exist at these points. New information has been obtained on the temperature dependence of the susceptibility and correlation length, cluster shape and anisotropy, and the critical exponents. Fiz. Tverd. Tela (St. Petersburg) 41, 148–152 (January 1999)     

Two-photon spectroscopy of ytterbium

Two-photon laser spectra of the Yb vapor have been obtained. Transitions to highly excited 4f¹⁴ 6sns¹S₀ and 4f¹⁴ 6snd ¹D₂ states are seen in direct two-photon excitation. Hybrid resonances involving 4f¹⁴ 6s6p ¹P⁰₁ and 4f¹⁴ 5d6s ³D₂ intermediate states lead to transitions to 4f¹⁴ 6sns¹S₀, 4f¹⁴ 6snp ³P⁰_2,1 and 4f¹⁴ 6snd ¹D₂ levels.     

Two-photon spectroscopy in AgCl

Two-photon absorption constant β⁽²⁾ has been measured in the indirect gap of AgCl between 4 and 4.4 eV. The absorption constant has been derived from the excitation spectrum of the recombination luminescence. The absolute value of β⁽²⁾ is in fair agreement with that calculated for a two-photon phonon-assisted transition.     

35Cl nuclear quadrupole resonance and the macroscopic superstructure in Hg2Cl2 crystal

A study has been made of the temperature dependences of the chlorine NQR frequency and linewidth in calomel crystals. These dependences show clearly pronounced anomalies in the region of the ferroelastic phase transition. The dynamic and static factors influencing the pattern of these dependences are discussed. A fine structure in the spin echo in the form of beats has been revealed within a narrow temperature interval near the phase transition. This effect is shown to be connected with the formation of a macroscopic heterophase superstructure in the crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1287–1290 (July 1997)     

Two-photon spectroscopy of antiprotonic helium

The precision of laser spectroscopy of antiprotonic helium (a helium atom with one of its electrons replaced by an antiproton) has improved by almost 4 orders of magnitude over its 20 years of history. Experimental transition frequencies can be compared to 3-body QED calculations to derive the antiproton-electron mass ratio. In the latest measurements of the Asacusa experiment at CERN, two-photon transitions of antiprotonic helium were excited using two counter-propagating laser beams. This method reduces the Doppler-broadening caused by the thermal motion of the atoms, and allowed us to measure the transition frequencies with a fractional precision of 2.5–5 parts in 10⁹. From these frequencies, we derived an antiproton-electron mass ratio of 1836.1526736(23). Our precision approaches that of the experimental value of the proton-electron mass ratio, and agrees with the latter within errors. Assuming CPT symmetry (i.e. \(m_{p}=m_{\overline {p}}\) ), we further derived the electron’s atomic mass as m _e = 0.0005485799091(7)u from the more accurately known atomic mass of the proton.     

Two-photon standing-wave fluorescence correlation spectroscopy

A fluorescence correlation spectroscopy experiment that combines two-photon excitation and a standing-wave interference pattern is presented. The experimental correlation function can be analyzed using a simple expression involving (1) an exponential decay with time constant tau(f), which reflects diffusion across the interference fringes, and (2) a longer-lived decay with time constant tau(omega), which reflects diffusion in and out of the focal spot. The diffusion of Rhodamine 110 in water and ethylene glycol is measured using this method. The ability to simultaneously measure diffusion on two different time and lengthscales makes this experiment especially useful in environments where anomalous diffusion is suspected.     

Two-photon photo-voltaic spectroscopy on wannier excitons in Cu2O

Two-photon excitation spectra of photo-voltaic effects and relevant excitonic emissions of a naturally grown single crystal of Cu₂O have been studied at 77 K and 2 K. The photo-voltaic effects have been detected under local excitation at a macroscopical distance apart from the electrodes. The participation of the yellow series excitons in the generation of the photo-voltaic signal has been discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 921–923 (May 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Properties of 397 nm luminescent band of Hg2Cl2 single crystal

Measurement of emission and excitation spectra of the luminescent band peaked at 397 nm of the mercurous chloride single crystals has been performed. This emission has been observed below the phase transition temperature. From the observed azimuthal dependence of the degree of polarization of the luminescence it was concluded that the luminescent centres are oriented in [001] direction. It is assumed that the luminescent centres are formed by Hg⁰ atoms, which interact strongly in the excited state with the surrounding ions of chlorine.The authors are much indebted to Dr. A.Bohun for his interest in this work and for many useful discussions.     

Two-photon spectroscopy of SnO2 under hydrostatic pressure

The pressure shift of S excitons in the rutile-type semiconductor tin oxide (SnO₂) is measured by two-photon absorption. From these data the pressure coefficients of the band gap (62.0 meV/GPa) and of the exciton binding energy (0.87 meV/GPa) are determined.