Y. R. Sun

THE v = 3 ← 0 S(0)-S(3) ELECTRIC QUADRUPOLE TRANSITIONS OF H2 NEAR 0.8 μm

The very weak S(0)-S(3) electric quadrupole transitions of the second overtone band of molecular hydrogen have been recorded in the laboratory by continuous-wave cavity ring-down spectroscopy near 0.8 μm. The ultrahigh sensitivity of the spectrometer (αmin ~ 1 × 10–10 cm–1) allows us to detect the considered transitions at a relatively low sample pressure (50-750 torr). The line positions, intensity, and pressure-shift coefficients are derived from a fit of the line shape using a Galatry profile. Compared with literature values, the relative differences between the experimental and theoretical transition intensities are reduced by one order of magnitude, reaching a value of about 2% mainly dependent of the line-shape function adopted for the profile fitting. The thermal equilibrium relative intensity of the S(1) to S(0) line is determined with an accuracy of 0.4%, which can be used to probe the ortho- to para-H2 concentration ratio. Our measurements confirm the quality of the high-level ab initio calculations, including the relativistic and quantum electrodynamics corrections.

Line Parameters of the 782 nm Band of CO2

The 782 nm band of CO{sub 2}, in a transparent window of Earths atmosphere, was the first CO{sub 2} band observed 80 yr ago in the spectra of Venus. The band is very weak and therefore not saturated by the thick atmosphere of Venus, but its spectral parameters are still very limited due to the difficulty of detecting it in the laboratory. It is the highest overtone (ν{sub 1} + 5ν{sub 3}) of CO{sub 2} given in widely used spectroscopy databases such as HITRAN and GEISA. In the present work, the band is studied using a cavity ring-down spectrometer with ultra-high sensitivity as well as high precision. The positions of 55 lines in the band were determined with an absolute accuracy of 3 × 10{sup –5} cm{sup –1}, two orders of magnitude better than previous studies. The line intensities, self-induced pressure broadening coefficients, and the shift coefficients were also derived from the recorded spectra. The obtained spectral parameters can be applied to model the spectra of the CO{sub 2}-rich atmospheres of planets like Venus and Mars.

Laser Spectroscopy of the Fine-Structure Splitting in the 2PJ3 Levels of He4

The fine-structure splitting in the 2^{3}P_{J} (J=0, 1, 2) levels of ^{4}He is of great interest for tests of quantum electrodynamics and for the determination of the fine-structure constant α. The 2^{3}P_{0}-2^{3}P_{2} and 2^{3}P_{1}-2^{3}P_{2} intervals are measured by laser spectroscopy of the ^{3}P_{J}-2^{3}S_{1} transitions at 1083xa0nm in an atomic beam, and are determined to be 31u2009908u2009130.98±0.13u2009u2009kHz and 2u2009291u2009177.56±0.19u2009u2009kHz, respectively. Compared with calculations, which include terms up to α^{5}Ry, the deviation for the α-sensitive interval 2^{3}P_{0}-2^{3}P_{2} is only 0.22xa0kHz. It opens the window for further improvement of theoretical predictions and an independent determination of the fine-structure constant α with a precision of 2×10^{-9}.

Comb-locked cavity ring-down saturation spectroscopy

We present a new method of comb-locked cavity ring-down spectroscopy for the Lamb-dip measurement of molecular ro-vibrational transitions. By locking both the probe laser frequency and a temperature-stabilized high-finesse cavity to an optical frequency comb, we realize saturation spectroscopy of molecules with kilohertz accuracy. The technique is demonstrated by recording the R(9) line in the υ = 3 - 0 overtone band of CO near 1567 nm. The Lamb-dip spectrum of such a weak line (transition rate 0.0075 s-1) is obtained using an input laser power of only 3 mW, and the position is determined to be 191 360 212u2009770 kHz with an uncertainty of 7 kHz (δν/ν∼3.5×10-11), which is currently limited by our rubidium clock.

Measurement of the Frequency of the 2S3−2P3 Transition of He4

The 2u2009^{3}S-2u2009^{3}P transition of ^{4}He was measured by comb-linked laser spectroscopy using a transverse-cooled atomic beam. The centroid frequency was determined to be 276u2009736u2009495u2009600.0(1.4)xa0kHz, with a fractional uncertainty of 5.1×10^{-12}. This value is not only more accurate but also differs by as much as -49.5u2009u2009kHz (20σ) from the previous result given by [Cancio Pastor et al., Phys. Rev. Lett. 92, 023001 (2004)PRLTAO0031-900710.1103/PhysRevLett.92.023001; Cancio Pastor et al.Phys. Rev. Lett.97, 139903(E) (2006)10.1103/PhysRevLett.97.139903; Cancio Pastor et al.Phys. Rev. Lett.108, 143001 (2012)10.1103/PhysRevLett.108.143001]. In combination with ongoing theoretical calculations, this work may allow the most accurate determination of the nuclear charge radius of helium.

Ultrasensitive, self-calibrated cavity ring-down spectrometer for quantitative trace gas analysis

A cavity ring-down spectrometer is built for trace gas detection using telecom distributed feedback (DFB) diode lasers. The longitudinal modes of the ring-down cavity are used as frequency markers without active-locking either the laser or the high-finesse cavity. A control scheme is applied to scan the DFB laser frequency, matching the cavity modes one by one in sequence and resulting in a correct index at each recorded spectral data point, which allows us to calibrate the spectrum with a relative frequency precision of 0.06xa0MHz. Besides the frequency precision of the spectrometer, a sensitivity (noise-equivalent absorption) of 4×10-11u2009u2009cm-1u2009u2009Hz-1/2 has also been demonstrated. A minimum detectable absorption coefficient of 5×10-12u2009u2009cm-1 has been obtained by averaging about 100 spectra recorded in 2 xa0h. The quantitative accuracy is tested by measuring the CO2 concentrations in N2 samples prepared by the gravimetric method, and the relative deviation is less than 0.3%. The trace detection capability is demonstrated by detecting CO2 of ppbv-level concentrations in a high-purity nitrogen gas sample. Simple structure, high sensitivity, and good accuracy make the instrument very suitable for quantitative trace gas analysis.

Precision spectroscopy of the electronic ground state of the hydrogen molecule

H2 is the most abundant molecule in the universe. It is also the only neutral molecule whose energy levels can be precisely calculated based on quantum electrodynamics and fundamental physical constants without using any adjustable parameters. Precision spectroscopy of the hydrogen molecule is of great importance in astrophysical observation and in testing fundamental physics as well. Here we review the theoretical and experimental spectroscopy studies of the electronic ground state of the hydrogen molecule. The frequency of the R(1) transition in the V =2-0 band of HD is also determined with an accuracy of 5×10−10, which is the most precisely measured ro-vibrational transition of the hydrogen molecule to date. By comparing the calculated and experimental results, we present perspectives of the precision spectroscopy of the hydrogen molecule in the determination of fundamental physical constants such as the proton-electron mass ratio.