Doppler-free Two-photon Excitation Research Articles

High-resolution two-photon spectroscopy of a 5p56p←5p6 transition of xenon

We report high-resolution Doppler-free two-photon excitation spectroscopy of Xe from the ground state to the $5{p}^{5}(^{2}P_{3/2})6p\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}^{2}[3/2]_{2}$ electronic excited state. This is a first step to developing a comagnetometer using polarized $^{129}\mathrm{Xe}$ atoms for planned neutron electric dipole moment measurements at TRIUMF. Narrow linewidth radiation at 252.5 nm produced by a continuous wave laser was built up in an optical cavity to excite the two-photon transition, and the near-infrared emission from the $5{p}^{5}6p$ excited state to the $5{p}^{5}6s$ intermediate electronic state was used to detect the two-photon transition. Hyperfine constants and isotope shift parameters were evaluated and compared with previously reported values. In addition, the detected photon count rate was estimated from the observed intensities.

Formula omitted] and [formula omitted] level energies of D2

Accurate absolute level energies of the B1Σu+, v=0–8, N and EF1Σg+, v=0–21, N rovibrational quantum states of molecular deuterium are derived by combining results from a Doppler-free two-photon laser excitation study on several lines in the EF1Σg+–X1Σg+ (0,0) band, with results from a Fourier-transform spectroscopic emission study on a low-pressure hydrogen discharge. Level energy uncertainties as low as 0.0005cm−1 are obtained for some low-lying E1Σg+ inner-well rovibrational levels, while uncertainties for higher-lying rovibrational levels and those of the F1Σg+ outer-well states are nominally 0.005cm−1. Level energies of B1Σu+ rovibrational levels, for v⩽8 and N⩽10 are determined at an accuracy of 0.001cm−1. Computed wavelengths of D2 Lyman transitions in the B1Σu+–X1Σg+ (v,0) bands are also tabulated for future applications.

Accurate level energies in the EF1 , GK1 , H1 , B1 , , B′1 , , , J1Δg states of H2

By combining results from a Doppler-free two-photon laser excitation study on several lines in the EF1 − X1 (0,0) band of H2 with results from a Fourier-transform spectroscopic study on a low-pressure discharge in hydrogen, absolute level energies, with respect to the X1 , v = 0, N = 0 ground level, were determined for 547 rovibronically excited states in H2. While for some of the levels in the EF1 and B1 states the uncertainties are as low as 0.0001 cm−1, the accuracy of other levels is lower. The general improvement in the accuracy for the comprehensive data set of level energies is by an order of magnitude with respect to previous measurements. An updated listing of transition wavelengths of the spectral lines in the Lyman and Werner bands is presented, based on combination differences between the presently obtained B1 and level energies and those in the X1 ground state.

Isotope-selective excitation of ^41Ca isotope in Doppler-free two-photon continuous-wave excitation: a case study

Seven schemes are studied theoretically for Doppler-free two-photon excitation of rare (41)Ca isotope using single-mode continuous-wave lasers. The ionization efficiencies and optical selectivities for all the schemes are calculated for various powers of the excitation and ionization lasers and for various focusing conditions of the two lasers. To maximize the ionization efficiencies and the optical selectivities, wavelength-dependent Stark compensation is used. Certain laser wavelengths of the ionization step termed as magic wavelengths are identified for compensating the Stark shift induced by the excitation laser. The effects of the Stark-shift-induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities are investigated. The ionization efficiency and optical selectivity for the best scheme after Stark compensation are found to be 8.4 x 10(-4) and approximately 9 x 10(3), respectively.

Determinations ofEFΣg+1←XΣg+1transition frequencies inH2,D2, andHD

We have measured the energies of several rotational branches of the $(0,0)$ band of the $EF$ $^{1}\ensuremath{\Sigma}_{g}^{+}\ensuremath{\leftarrow}X^{1}\ensuremath{\Sigma}_{g}^{+}$ transition in molecular hydrogen by use of Doppler-free two-photon laser excitation at $202\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The accuracy for all three stable isotopic variations is 8 parts in ${10}^{9}$, nominally a fourfold improvement over previously available results, but some of the transition energies differ from previous work by as much as three standard deviations. The improved accuracy derives principally from our ability to measure the optical phase evolution of nanosecond laser pulses, then to predict quantitatively the line shapes of multiphoton transitions excited by them. The results allow comparably accurate determinations of the $EF$ state term energies, which in turn help to calibrate the entire excited-state spectrum of this fundamental molecular system. These improved calibrations are particularly useful to recent and ongoing efforts to determine the dissociation energy and ionization potential with improved accuracy.

Doppler-Free Two-Photon Excitation Spectroscopy and the Zeeman Effect in the “Channel Three” Region of C6H6

Abstract Doppler-free two-photon excitation spectra and the Zeeman effects for the 1021401 and 1011401 bands, whose vibrational excess energies are 3412 and 2492 cm−1, respectively, of the S11B2u ← S01A1g transition in gaseous benzene-h6 have been measured. Rotationally resolved lines up to a rotational excess energy of 945 cm−1 in the 1011401 band, whose excess energy is isoenergetic with the one of the 1021401 band at low J, have been observed. The density of perturbation is observed to increase around the K = 0 and K = J levels in the 1011401 band as the rotational energy increases, but line broadening is not observed. It was reported that only the K = 0 lines at low J and the K = J lines at high J were observed as a sharp line and the rest was washed out by broadening in the 1021401 band. The assignments are confirmed by the Zeeman spectra. The character and magnitude of Zeeman splittings in both bands could be understood as originating from the electronic orbital angular momentum arising from a mixing of the S11B2u and S21B1u states via J–L coupling. The levels of the S11B2u state are found not to be mixed with a triplet state by the Zeeman effect.

Doppler-Free Two-Photon Excitation Spectroscopy and the Zeeman Effects. Perturbations in the and Bands of the S1← S0Transition of C6D6

Doppler-free two-photon excitation spectra and the Zeeman effects for the 1 band of the S1 1B2u <-- S0 1A1g transition in gaseous benzene-d6 were measured. Although the spectral lines were strongly perturbed, almost all of the lines near the band origin could be assigned. From a deperturbation analysis, the perturbation near the band origin was identified as originating from an anharmonic resonance interaction. Perturbation centered at K = 28-29 in the 14(0)1 band was analyzed, and it was identified as originating from a perpendicular Coriolis interaction. The symmetry and the assignment of the perturbing state proposed by Schubert et al. (Schubert, U.; Riedle, E.; Neusser, H. J. J. Chem. Phys. 1989, 90, 5994.) were confirmed. No perturbation originating from an interaction with a triplet state was observed in both bands. From the Zeeman spectra and the analysis, it is demonstrated that rotationally resolved levels are not mixed with a triplet state. The intersystem mixing is not likely to occur at levels of low excess energy in the S1 state of an isolated benzene. Nonradiative decay of an isolated benzene in the low vibronic levels of the S1 state will occur through the internal mixing followed by the rotational and vibrational relaxation in the S0 state.

Doppler-free two-photon excitation spectroscopy and the Zeeman effects of the S1B1u1(v21=1)←SAg1(v=) band of naphthalene-d8

Doppler-free two-photon excitation spectrum and the Zeeman effect of the S1 1B1u(v21=1) <-- S0 1Ag(v=0) transition of naphthalene-d8 have been measured. 908 lines of Q(Ka)Q(J)KaKc transition of J=0-41, Ka=0-20 were assigned, and the molecular constants of the S1 1B1u(v21=1) state were determined. Perturbations were observed, and those were identified as originating from Coriolis interaction. No perturbation originating from an interaction with triplet state was observed. The Zeeman splittings from lines of a given J were observed to increase with Kc, and those of the Kc=J levels increased linearly with J. The Zeeman effects are shown to be originating from the magnetic moment of the S1 1B1u state, which is along the c axis and is induced by mixing of the S2 1B3u state to the S1 1B1u state by J-L coupling. Rotationally resolved levels were found not to be mixed with a triplet state from the Zeeman spectra. Accordingly, it is concluded that nonradiative decay of an isolated naphthalene excited to low rovibronic levels in the S1 1B1u state does not occur through the intersystem mixing. This is at variance with generally accepted understanding of the pathways of the nonradiative decay.

Doppler-free two-photon excitation spectroscopy and the Zeeman effect of the 141 band of the S1 1B2u←S 1A1g transition of benzene-d6

The Doppler-free two-photon excitation spectrum and the Zeeman effect of the 1401 band of the S1←S0 transition of C6D6 were measured from 39 842 to 39 856 cm−1. The Zeeman splittings of the Q(J)Q(K) lines of a given J were observed to increase proportionally with K2 and reach a maximum at K=J. The Zeeman splittings of the Q(J)Q(K=J) lines were observed to increase proportionally with J. The observed Zeeman splittings are identified as originating from the electronic orbital angular momentum arising from a mixing of the S1 1B2u and S2 1B1u states via J-L coupling. No perturbation originating from an interaction with a triplet state was observed. It became clear from the Zeeman effect that rotationally resolved levels of the S1 state are not mixed with a triplet state. Therefore, it is concluded that nonradiative decay of an isolated benzene molecule excited to the S1 state occurs through the intramolecular vibrational-rotational mixing within the S1 state even in the low vibrational levels.

Zeeman spectra of the à 1Au←X̃ 1Ag transition of trans-glyoxal studied by Doppler-free two-photon fluorescence excitation spectroscopy

Doppler-free two-photon fluorescence excitation spectra of the A 1Au(v7′=1)←X 1Ag(v″=0) transition of trans-glyoxal were measured by propagating a laser beam either polarized parallel (π pump) or perpendicular (σ pump) to the magnetic field of 6 T. π pump and σ pump dependence of the Zeeman spectra of the Q(K)Q(J=6) lines for K=0–6 was measured. Intensities of the Zeeman components of the Q(6)Q(6) lines were observed to have the maxima at the high and low wave number ends and the minimum at the middle for π pump, and the minima at the high and low wave number ends and the maximum at the middle for σ pump. By comparing calculated and observed patterns of the Zeeman spectra, it became clear that the transition tensor M00 is dominant and the effective intermediate state is 1 1Bu(ππ*). K dependence of the Zeeman spectra of the Q(K)Q(J=15) lines was measured, and the Zeeman splittings were observed to increase proportionally to K2. J dependence of the Zeeman spectra of the Q(K=J)Q(J) lines was measured for J=...

Doppler-free two-photon excitation of Lyman-α fluorescence for the diagnostics of magnetically confined fusion plasmas

Doppler-free two-photon excitation of hydrogen Lyman-α fluorescence is investigated as a possible laser-induced fluorescence (LIF) technique for the diagnosis of magnetically confined fusion (MCF) plasmas. A formal analysis is presented of the underlying atomic and plasma physics as well as of various practical aspects, such as parameter optimization and experimental precision. The latter is analyzed with regard to the photon noise and to the sensitivity of the fluorescence signals to the plasma and laser parameters. The diagnostic potential of the LIF technique described relies on its high spectral resolution. Thus, the absorption lines of the hydrogen isotopes H, D, and T are clearly separated from each other and can serve for isotope-selective density measurements. In addition, using a tunable laser system with small bandwidth, various plasma parameters can be inferred from the spectral line shapes, such as the neutrals’ temperatures or the effective charge number Zeff. The polarization of the fluorescence can, under favorable circumstances, be exploited for magnetic field measurements. The photon statistics impose neutral densities above 1014 m−3 and thus make the diagnostic suitable primarily for the plasma edge. However, previous work has shown that it is applicable even in the plasma bulk of large machines if a neutral beam is used that generates sufficient neutral densities by charge exchange with the plasma ions. Variations or insufficient knowledge of the neutrals’ temperatures are found to seriously affect the precision of absolute isotope density measurements. They are rather uncritical, however, for the determination of the H/D/T density ratios, which are of prime importance for the burn control of large MCF devices. A notable sensitivity is also found to variations of the laser frequency detuning, which should be known and stable to better than 100 MHz. The perspectives demonstrated in the present study and the success of a first experiment on the plasma generator PSI 1 at IPP Berlin are considered to be sufficient motivation for testing the diagnostic on a tokamak or other MCF device.

Measurement of the deuterium/tritium fuel mixture in magnetic confinement fusion plasmas by Doppler-free two-photon spectroscopy

Doppler-free two-photon excitation of fluorescence may be used to measure the deuterium/tritium fuel mix in magnetic confinement fusion plasmas. Two atomic transition schemes of the hydrogen isotopes come into question for this purpose: (I) excitation of the first excited energy level (n=2) and observation of Lyman-α, (II) excitation of the second excited energy level (n=3) and observation of Balmer-α. Comparison of the two schemes indicates that both may have their merits in particular situations. Due to lack of information about the future laser characteristics achievable for scheme II, the detailed performance of this scheme cannot yet be definitely assessed.

Sensitive Electric Field Measurement by Fluorescence-Dip Spectroscopy of Rydberg States of Atomic Hydrogen

Fluorescence-dip spectroscopy based on an optical double resonance is used to probe the Stark splitting in highly excited Rydberg states of atomic hydrogen. After Doppler-free two-photon excitation to $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$, fluorescence at Balmer $\ensuremath{\alpha}$ is observed. When the radiation of a second laser is tuned over the resonance between $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ and a Rydberg state, the Stark splitting manifests itself as a series of dips in the Balmer- $\ensuremath{\alpha}$ fluorescence intensity. Rydberg states up to $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}55$ can be identified. The minimum detectable electric field is $5\mathrm{V}/\mathrm{cm}$. This is about an order of magnitude more sensitive than other laser-induced fluorescence techniques.

Two-photon spectroscopy: A technique for characterizing diode-laser noise.

Using Doppler-free Rb two-photon excitation measurements, we present support for the hypothesis that semiconductor-laser noise is characterized by pure phase-diffusion noise. We measured near-Lorentzian shapes for both the laser and the two-photon excitation spectra and a slope of 3.7\ifmmode\pm\else\textpm\fi{}0.3 for the dependence of the two-photon excitation width on the laser width. These represent measurements of the second-order field statistics of a naturally operating laser where the noise is dominated by spontaneous emission. The measured spectral shape and slope are in excellent agreement with the Mollow model, which predicts a Lorentzian spectral shape and a slope of 4 for weak-field, two-photon excitation with a pure phase-diffusion field (Lorentzian spectral density). The dominance of phase-diffusion noise is further corroborated by an analytically solvable microscopic noise model that includes phase and amplitude noise.

Second-order magnetic contributions to the hyperfine splitting of the 5snd 1D2 states in 87Sr.

The hyperfine structure of 5snd ^1D_2 states in ^{87}Sr was studied for principal quantum numbers 11 <= n <= 25 by Doppler-free two-photon excitation combined with thermionic detection. Based on the more accurate data, compared to previous results, the hyperfine structure was analysed by second order perturbation theory using MQDT wavefunctions. As a result we find that the surprisingly large B-factors obtained by fitting the data with the Casimir formula do not reflect a quadrupolar interaction but rather consist predominantly of second order magnetic dipole contributions. Further, inconsistencies in the King plot of odd-even isotope shifts were removed by this type of analysis. The result demonstrates the importance of higher order effects in hyperfine splittings of excited states in atoms with two valence electrons, even when the fine structure splitting is large compared to the hyperfine splitting.

Hyperfine-structure studies of high-lying even-parity levels of ^139La i by resonantly enhanced Doppler-free two-photon spectroscopy

The technique of employing resonantly enhanced Doppler-free two-photon excitation from either the ground state or a low-lying metastable state for precisely measuring the hyperfine structure of the atomic high-lying levels has been used, for the first time to our knowledge, to study 139La i. By detecting the laser-induced fluorescence produced in a hollow-cathode discharge tube, one can measure a set of spectra for each two-photon transition line. The spectra are analyzed in detail and fitted to yield hyperfine structure constants. The A values of eight high-lying even-parity levels of 139La i are then determined.

A dye laser spectrometer for high resolution spectroscopy

Abstract A high resolution dye laser spectrometer is described. The laser frequency is locked to a resonance frequency of a high finesse Fabry-Perot cavity by a sideband modulation technique. The residual frequency fluctuations of the stabilized laser were measured with respect to an independent frequency discriminator. From this measurement a value of the Allan variance of 10 -14 at sampling times ranging between 10 -5 and 1 s was derived, indicating a laser linewidth of 10 Hz. By Doppler-free two-photon excitation of rubidium Rydberg states we observed optical Ramsey fringes with a linewidth (fwhm) of 7.5 kHz limited by the time-of- flight of the atoms between the separated interaction zones.

Two-photon spectroscopy of some even-parity levels in neutral ytterbium

Using Doppler-free two-photon excitation from the ground state 4f146s2 1S0 the even-parity J=2 levels of the 4f146s7d and 4f146s8d configurations as well as the 4f146p2, 4f136s26p and 4f135d6s6p configurations between 44760.4 and 47341.8 cm-1 in neutral ytterbium have been investigated with regard to the hyperfine structure of the excited states and the isotope shift of these transitions. The experimental results for the hyperfine splitting constants have been analysed regarding possible singlet-triplet and configuration mixing. The experimental values have been compared with theoretical results obtained in an ab initio calculation. A strong influence for the second-order hyperfine interaction could be established in both the 4f146s7d 3D2 and the 4f146s10s 1S0 states.

Theory of anharmonically modified Coriolis coupling in the S1 state of benzene and relation to experiment

Avoided crossings between quasidegenerate rovibrational states in the Doppler-free two-photon excitation of the 141 mode in the S1 excited state of benzene are treated theoretically. Two sets of avoided crossings in plots of spectral line frequency vs J at a given K and ΔK have been reported experimentally between an initially prepared ‘‘light’’ state (141 in zeroth order) and dark states, namely, one which in zeroth order is a 51101161 state, the other being in zeroth order a 62111 and/or possibly a 31161 state, implicated earlier by Neusser et al. The identification of these states makes the phenomenon an excellent candidate for treatment of the avoided crossing via a Van Vleck transformation, no other basis set states being needed for the diagonalization in order to extract the important features. Two successive transformations are used for handling direct coupling and coupling via virtual states. The dominant calculated contribution to the coupling is, jointly, Coriolis plus cubic–cubic anharmonic interactions between vibrational modes. Playing less of a role are Coriolis terms in which the inverse moment of inertia tensor is expanded up to quadratic terms in the coordinates. There results a 5×5 (for coupling to 51101161 ) and a 3×3 (for coupling to 62111 or 31161 ) matrix of the transformed Hamiltonian, each of which can also be described, if desired, to a very good approximation by a 2×2 matrix. The coupling element V0 and the difference of the rotational constants for the light and dark states (ΔB) are obtained from the plots of line position vs J(J+1) obtained. For the 141 to 51101161 and for the 141 to 62111 couplings the theoretical results are in reasonable agreement with the experimental results, no adjustable parameters being employed. For a coupling of 141 to 31161 the calculated V0 would be much too high compared with experiment (a factor of 10), the coupling involving the exchange of only three instead of four vibrational quanta. A situation in which the 141 state is coupled to the 62111 state to yield an avoided crossing and off-resonantly coupled to the 31161 state would be consistent with some experimental results and not affect the reasonable agreement of the slope difference and splitting for the avoided crossing plots.

Doppler-free two-photon-excited fluorescence spectroscopy of atomic hydrogen in flames

We describe Doppler-free 205-nm two-photon excitation studies of atomic hydrogen performed using a single-mode pulsed dye-laser system in low-pressure flames. Excitation spectra confirm calculated line-strength ratios for transitions to the 3S and 3D states. Collisional broadening rates are obtained in several flames and compared with quenching rates measured in the same flames, using time-resolved fluorescence measurements.