Hyperfine structure quantum beat spectroscopy of Tl statesn 2 P 3/2 (n=10−13) with pulsed two-photon excitation

By quantum beat spectroscopy with pulsed two-photon excitation the hyperfine constants of the Thallium states 6s ² np ² P _3/2 (n=10?13) are determined: 2A(10P _3/2)=77.73 (74) MHz, 2A(11P _3/2)=48.82 (49) MHz, 2A(12P _3/2)=32.34 (45) MHz and 2A(13P _3/2)=23.08 (35) MHz.     

The 3d states of23Na and7Li: determination of unresolved hyperfine splittings and radiative lifetimes

The electronic transitions from the ground state to the 3d states have been studied for²³Na and⁷Li atoms by two-photon Doppler-free laser spectroscopy. The positions of the resonances are used to determine the unresolved hyperfine structure of the 3d states from which the magnetic-dipole hyperfine interaction constantsA are derived. The results for theA factors are:A(3d ² D _3/2;²³Na)=+527(25) kHz;A(3d ² D _5/2;²³Na)=+108.5(2.4) kHz;A(3d ² D _3/2;⁷Li)=+843(41) kHz;A(3d ² D _5/2;⁷Li)=+343.6(1.0) kHz. For the fine structure intervals fs of the 3d doublets we obtain: fs(3d Na)=-1 494 444(44) kHz and fs(3d Li)=+1 083 936(60) kHz. The linewidths of the resonances are evaluated with respect to the natural lifetimes of the 3d states. For Na the result is τ(3d Na)=19.27(23) ns.     

Multiphoton ionization and two-photon excitation spectroscopy of nitric oxide

The multiphoton ionization and two-photon excited fluorescence action spectra of nitric oxide are compared. At low fluence the two spectra differ when more than three photons are required for ionization. This may be due to the presence of several energy acquisition routes, providing new spectroscopic information on intermediate slates.     

Predissociation dynamics of Ã-state ammonia probed by two-photon excitation spectroscopy

Electronically excited ND₃(A¯) molecules have been prepared by laser two-photon excitation on theA¯¹A″₂—X¯¹′₁ transition and monitored via their resulting short-lived emission. The earlier observation of Douglas that ND₃(A¯) molecules carrying one quantum of out-of-plane bending vibration ν′₂ are least susceptible to predissociation, is confirmed. ND₃(A¯) predissociation rates are found to be both vibronic and rovibronic level dependent. Both observations may be understood by considering the likely form of the potential energy surface for ND₃(A¯) molecules in the region of the D₂N—D dissociation coordinate. At short D₂N—D separations this surface exhibits a barrier. The presence of a conical intersection (involving the ND₃ ground state surface) further out along the dissociation coordinate has a crucial influence on the magnitude of this barrier. The envisaged form of theA¯-state potential energy surface also provides a qualitative rationale for all previous experimental findings concerning electronic branching ratios and energy disposal amongst the primary photofragments arising in the photodissociation ofA¯-state ammonia.     

Protonation and conformational dynamics of GFP mutants by two-photon excitation fluorescence correlation spectroscopy

GFP mutants are known to display fluorescence flickering, a process that occurs in a wide time range. Because serine 65, threonine 203, glutamate 222, and histidine 148 have been indicated as key residues in determining the GFP fluorescence photodynamics, we have focused here on the role of histidine 148 and glutamate 222 by studying the fluorescence dynamics of GFPmut2 (S65A, V68L, and S72A GFP) and its H148G (Mut2G) and E222Q (Mut2Q) mutants. Two relaxation components are found in the fluorescence autocorrelation functions of GFPmut2: a 10-100 micros pH-dependent component and a 100-500 micros laser-power-dependent component. The comparison of these three mutants shows that the mutation of histidine 148 to glycine induces a 3-fold increase in the protonation rate, thereby indicating that the protonation-deprotonation of the chromophore occurs via a proton exchange with the solution mediated by the histidine 148 residue. The power-dependent but pH-independent relaxation mode, which is not affected by the E222Q and H148G mutations, is due to an excited-state process that is probably related to conformational rearrangements of the chromophore after the photoexcitation, more than to the chromophore excited-state proton transfer.     

Experimental study of the 39K2 2 3Pi(g) state by perturbation facilitated infrared-infrared double resonance and two-photon excitation spectroscopy

The 39K2 2 3Pi(g) state has been observed by perturbation facilitated infrared-infrared double resonance and two-photon excitations. The vibrational numbering of the 2 3Pi(g) levels was determined by resolved fluorescence into the bound levels as well as to the continuum of the a 3Sigma(u)+ state. The rotational assignment of the 2 3Pi(g) levels excited by two-photon transitions was determined from excitation frequencies and resolved fluorescence into the bound levels of the a 3Sigma(u) + and b 3Pi(u) states. Molecular constants obtained from these observed levels agree with theoretical constants.     

The structure and hyperfine splittings of simple phosphoranyl radicals

The ab initio UHF method has been employed to calculate equilibrium geometries and isotopic hyperfine coupling constants for the radicals PH₂, PF₂, PH₄ and PF₄.     

Measurement of hyperfine structure of sodium 3P 1/2,3/2 states using optical spectroscopy

The hyperfine levels of the sodium 3P _1/2,3/2 states were resolved using a narrow linewidth laser to excite the ground state. The laser frequency was scanned while fluorescence resulting from the radiative decay of the excited state was detected. The frequency was calibrated using the known hyperfine splitting of the ground state. The magnetic dipoleA and electric quadrupoleB hyperfine coupling constants of the excited states were determined to be $A_{3P_{1/2} } = 94.44 \pm 0.13$ , $A_{3P_{3/2} } = 18.62 \pm 0.21$ and $B_{3P_{1/2} } = 2.11 \pm 0.52MHz$ . The uncertainty of $A_{3P_{1/2} } $ is less than results previously reported while the data for the 3P _3/2 state are consistent with those existing in the literature.     

Tl2Te and its relationship with Tl5Te3

The crystal structure of Tl₂Te, dithallium telluride, has been determined by single‐crystal X‐ray diffraction. The analysis of the structure shows that this compound is the first known representative of a new crystal structure type. The structural relationship with the related Tl₅Te₃ phase is discussed.     

Determination of asymmetry splittings in the polyatomic molecule propynal by Stark quantum beat spectroscopy

Coherences among asymmetry-split rotational levels in a molecule can be created when a weak electric field is applied. The quantum beats superimposed on the time-resolved fluorescence decay are utilized for the accurate measurement of asymmetry splittings. The technique is exemplified for selected vibronicS ₁ states of propynal and αD-propynal and the results are compared with conventional (i.e. frequency domain) spectroscopic data. The applicability of the presented method of coherence spectroscopy is discussed.     

Determination of the dipole moment of thioformaldehyde in the a3A2 excited state by laser phosphorescence excitation spectroscopy

Doppler-limited phosphorescence excitation spectra have been recorded at various electric fields for two rotational transitions in the $\text{a}$³A₂-$\text{X}$¹A₁ 0-0 band of H₂CS. Stark splittings were resolved, and were used to determine the dipole moment in the excited electronic state. The value found, 0.57(3) D, is of the order expected by comparison with dipole moments determined for other states of H₂CS, but rather lower than that predicted by ab initio calculations.     

Hydrogen bond lifetimes and energetics for solute/solvent complexes studied with 2D-IR vibrational echo spectroscopy

Weak pi hydrogen-bonded solute/solvent complexes are studied with ultrafast two-dimensional infrared (2D-IR) vibrational echo chemical exchange spectroscopy, temperature-dependent IR absorption spectroscopy, and density functional theory calculations. Eight solute/solvent complexes composed of a number of phenol derivatives and various benzene derivatives are investigated. The complexes are formed between the phenol derivative (solute) in a mixed solvent of the benzene derivative and CCl4. The time dependence of the 2D-IR vibrational echo spectra of the phenol hydroxyl stretch is used to directly determine the dissociation and formation rates of the hydrogen-bonded complexes. The dissociation rates of the weak hydrogen bonds are found to be strongly correlated with their formation enthalpies. The correlation can be described with an equation similar to the Arrhenius equation. The results are discussed in terms of transition state theory.     

A 340 nm emission system in I2 induced by a two-photon excitation process

Iodine vapour is optically excited in a two-step process populating selectively the first four vibrational levels of an ion-pair state having T_e = 47032 ± 6 cm^?1 and ω_e = 98 ± 2 cm^?1. Fluorescence from this state is observed at 340 nm with vibrational resolution and is found to terminate on levels of the intermediate B state.     

Determination of the dipole moment of thioformaldehyde in the A2 excited state by laser phosphorescence excitation spectroscopy

Doppler-limited phosphorescence excitation spectra have been recorded at various electric fields for two rotational transitions in the ³A₂- ¹A₁ 0-0 band of H₂CS. Stark splittings were resolved, and were used to determine the dipole moment in the excited electronic state. The value found, 0.57(3) D, is of the order expected by comparison with dipole moments determined for other states of H₂CS, but rather lower than that predicted by ab initio calculations.     

The structure of Tl2F2 from photoelectron spectroscopy

The photoelectron spectra of thallium fluoride monomer and dimer have been obtained. Two different oven techniques were used to generate the molecular beam. The resulting spectra of mixtures of the two species were separated into monomer and dimer components. A comparison of the Tl₂F₂ spectrum with semi-empirical molecular orbital calculations suggests that the dimer structure is rhombic.     

Precision measurements of hyperfine predissociation in I2 vapor using a two-photon resonant scattering technique

A two-photon resonant scattering technique is used to measure natural linewidths of individual hyperfine components of the ν′ = 43,J′ = 12 and 16 rovibrational levels in the I₂ B state (³π_ou⁺). Differences in the linewidths due to hyperfine predissociation are precisely determined. The predissociation constants and the radiative linewidth for ν′ = 43 are found to be [a_ν′]² = 3.36 ± 0.09 kHz, [c_ν′]² = 4.67 ± 0.51 Hz and Γ_r = 51 ± 4 kHz, respectively.     

Observation of spin splittings in the 2B1 state of no2 by means of polarization spectroscopy

Polarizition spectroscopy was applied to the analysis of the ²B₁ (K′ = 0)-²A, subbands of NO₂ at 474 and 455 nm. The lines of the     

Studies of lifetimes of rotationally cooled NO2 using time-resolved fluorescence excitation spectra

A taunble, pulsed dye laser with output in the region of 5750 to 6000 Å was used to excite rotationally cooled NO₂ which was produced by expansion in conjunction with argon carrier gas through a supersonic nozzle. The resulting time-resolved fluorescence excitation spectra were used the lifetimes of various vibronic bands of NO₂ (²B₂). The lifetimes measured were in the region of 15 to 40 μs which were shorter than those obtained from cell experiments. For each individual excitation wavelength, only a single exponential decay was observed from very early times through 250 μs.     

Measurement of excitation functions and radiative lifetimes of oxygen ions

Oxygen ions from a 2 MV tandem accelerator have been used in beam-foil spectroscopic studies of O II, O III and O IV in the wavelength region 2000–5000 Å. Excitation functions for relative level populations in O II, O III and O IV in the incident energy range of 1.2–3.75 MeV have been measured and interpreted in terms of the independent electron model. Mean radiative lifetime measurements for a number of levels including cascades 4631 Å feeding to 2450 Å and 4344 Å feeding to 2385 Å in O IV have resulted in consistent values. Lifetime measurements for some of the O II levels of astrophysical interest have been reinvestigated and significant deviations from earlier values were observed.     

The two-photon excitation spectrum of triphenylene in n-heptane single crystals

A comparison of the one-and two-photon absorption spectra of triphenylene in n-heptane single crystals at 1.6. K reveals that the lowest singlet state (S₁) is of ¹A′₁ symmetry while the second singlet state(S₂) has ¹A′₂ symmetry, in contradiction with the commonly accepted assignment.