Optical-optical Double-resonance Spectra Research Articles

Doppler-free dual-comb spectroscopy of Rb using optical-optical double resonance technique.

We present a Doppler-free high-resolution dual-comb spectroscopy technique in which a dual-comb system is employed to perform optical-optical double-resonance (OODR) spectroscopy. In our experimental study, Doppler-free high-resolution and high-frequency-accuracy broadband measurements were realized using the proposed OODR dual-comb spectroscopic technique, which does not require high-power-per-mode frequency combs. We observed fully resolved hyperfine spectra of 5P3/2 - 4D5/2, 4D3/2 transitions of Rb at 1530 nm and precisely determined the absolute frequencies of the transitions, with an uncertainty of less than 1 MHz. The variations of the OODR spectral line shapes due to power broadening and alignment and the effects of polarization on the dual-comb OODR spectra were also analyzed. This study provides a widely applicable technique for Doppler-free dual-comb spectroscopy of various gaseous species.

Optical-optical double resonance, laser induced fluorescence, and revision of the signs of the spin-spin constants of the boron carbide (BC) free radical.

The cold boron carbide free radical (BC X (4)Σ(-)) has been produced in a pulsed discharge free jet expansion using a precursor mixture of trimethylborane in high pressure argon. High resolution laser induced fluorescence spectra have been obtained for the B (4)Σ(-)-X (4)Σ(-) and E (4)Π-X (4)Σ(-) band systems of both (11)BC and (10)BC. An optical-optical double resonance (OODR) scheme was implemented to study the finer details of both band systems. This involved pumping a single rotational level of the B state with one laser and then recording the various allowed transitions from the intermediate B state to the final E state with a second laser by monitoring the subsequent E-X ultraviolet fluorescence. In this fashion, we were able to prove unambiguously that, contrary to previous studies, the spin-spin constant λ is negative in the ground state and positive in the B (4)Σ(-) excited state. It has been shown that λ″ < 0 is in fact expected based on a semiempirical second order perturbation theory calculation of the magnitude of the spin-spin constant. The OODR spectra have also been used to validate our assignments of the complex and badly overlapped E (4)Π-X (4)Σ(-) 0-0 and 1-0 bands of (11)BC. The E-X 0-0 band of (10)BC was found to be severely perturbed. The ground state main electron configuration is …3σ(2)4σ(2)5σ(1)1π(2)2π(0) and the derived bond lengths show that there is a 0.03 Å contraction in the B state, due to the promotion of an electron from the 4σ antibonding orbital to the 5σ bonding orbital. In contrast, the bond length elongates by 0.15 Å in the E state, a result of promoting an electron from the 5σ bonding orbital to the 2π antibonding orbitals.

Collisional energy transfer in the intermediate states used for optical–optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.

High-resolution laser spectroscopy of the potassium–argon molecule: The B2Σ + state

The absorption spectrum of the KAr molecule has been observed with high resolution between 13 032 and 13 077 cm −1 using tunable laser diodes as light sources, a supersonic beam for production of the molecules, and laser-induced fluorescence for detection. In addition, optical–optical double resonance (OODR) experiments have been performed to simplify the spectrum and to get rotational assignment. Altogether, 670 lines due to the transition B 2Σ + ← X 2Σ + have successfully been assigned with vibrational levels of the B state ranging from v = 0 to v = 6. The corresponding energy values were fitted to the well-known Dunham expansion. In addition, we were able to analyse a local perturbation between the vibrational level v = 1 of the B state and v = 14 of the A 2Π 3/2 state. Unexpected extra lines in the OODR spectra are most probably due to a collision-induced population of other levels. For the equilibrium distance and the well-depth of the B state we obtain from the Dunham expansion 7.03 (8) Å and 26.2 (8) cm −1, respectively.

Interatomic potentials of triplet s-Rydberg series of HgNe and HgAr van der Waals dimers

The optical–optical double resonance (OODR) spectra of Rydberg 3Σ+ states of Hg(n3S1)Ne (n=8–10) and Hg(83S1)Ar were measured by using A and B states as intermediate states in the OODR process. The interatomic potentials of three states of HgNe and one state of HgAr were determined over a wide range of interatomic distance, R=3–7 Å, by the analysis of the vibrational structure of their OODR spectra. It was found that the potential shape varies sensitively with n and converges to that of the ion core, HgNe+. Dissociation energies (De) of the Rydberg states for the n=8, 9, and 10 were derived to be 209(2), 284(2), and 309(2) cm−1, respectively. Using the quantum defect orbital [G. Simons, J. Chem. Phys. 60, 645 (1974)], which represents a hydrogenic radial wave function for a Rydberg state with a given quantum defect, was introduced to interpret the characteristic n dependence of the interatomic potential. It was shown that the interatomic potential for the Rydberg states can be expressed by the sum of the ion core potential, Vion(R), and the repulsive potential, Vex(R), which originates mainly from the exchange repulsion between the Rydberg electron and the attached rare gas atom. The interatomic potential for Hg(83S1)Ar, whose dissociation energy [De=1602(4) cm−1] is much deeper than that of Hg(83S1)Ne, was also interpreted consistently by expressing the potential as Vion(R)+Vex(R).

Possible rotationally resolved detection of NO2 in the C̃ 2A2 state by optical–optical double resonance spectroscopy

This Note describes the first rotationally resolved detection of NO2 in the C̃ 2A2 state by optical–optical double resonance (OODR) spectroscopy. The C̃ 2A2–X̃ 2A1 transition is symmetry forbidden. Direct experimental evidence of the C̃ 2A2 state had not been provided until the recent vibrationally resolved detection using photoelectron spectroscopy of NO−2 by Weaver et al. [J. Chem. Phys. 90, 2027 (1989)]. We have measured hundreds of the OODR spectra in the 500 ∼600 nm range and found that 11 rovibronic levels in the spectral region of 513.8∼516.2 nm belong to the Ka = 1 stack of a vibrational level of C̃ 2A2.

Optical–optical double resonance of NO2 in the region of 612–614 nm: The role of 2A1 vibronic levels as dark and perturbing state

An optical–optical double resonance (OODR) technique has been applied to the rotational analysis and vibronic assignment of NO2 absorption band in the region of 612–614 nm. The two step excitation through 2 2B2←2B2←X̃ 2A1 has allowed us to determine rotational quantum numbers (NKa,Kc) for 73 eigenstates with B2 vibronic symmetry, lying at 16 306–16 465 cm−1 above the ground state. Although they are severely perturbed and irregular in the rotational structure and spin doubling, we can classify the rovibronic levels as four stacks; two Ka=0 stacks with subband origins of 16 306.2 and 16 321.0 cm−1, and two Ka=1 stacks with origins of 16 312.5 and 16 326.0 cm−1. A near-prolate asymmetric top approximation is used to obtain the term values and rotational constants. Extraordinary large DN measured for 2B2 vibronic levels can be understood by well-known, strong vibronic coupling between à 2B2 and highly excited vibrational levels of X̃ 2A1. Among a number of perturbations observed, the spin–orbit (and/or orbital–rotation) coupling between ‘‘light’’ 2B2 and ‘‘dark’’ 2A1 vibronic levels is clearly shown for the first time by analyzing intensity patterns of the ν2 -scanned (second step) OODR spectra.

Fluorescence-detected optical-optical double resonance spectroscopy of Rydberg-Rydberg transitions in NO

Fluorescence-detected optical-optical double resonance (OODR) spectroscopy is used to determine the relative transition intensities from the 3pπ C 2Π, ν′=0 state to the n*= 6 supercomplex of NO. The results are in good agreement with the multi-channel quantum defect theory analysis of Fredin et al. (Mol. Phys. 60 (1987) 825) and support their conclusion that the intensities of the same transitions observed in their ionization-detected OODR spectra are strongly affected by predissociation.

Optogalvanic double-resonance spectroscopy in a neon discharge

A recently proposed method of optogalvanic double-resonance (OGDR) spectroscopy, which employs two cw lasers modulated at different frequencies and a dc discharge, has been investigated theoretically as well as experimentally. The technique is demonstrated to provide a versatile, state-selective spectroscopic method. A detailed analysis has been carried out for different situations where two lasers pump transitions which have the lower and/or upper state, or no state, in common. It also shows that the OGDR signal detected at the sum or difference modulation frequency can be given as a linear combination of the perturbed population densities in a plasma which give rise to the single-resonance signal. In addition, the results show that the OGDR signal polarity is characteristic for each type of double resonance. This has been verified in an OGDR experiment using a hollow-cathode discharge in neon and two cw dye lasers. State-selective OGDR spectra have been compared with optical-optical double-resonance spectra. Useful applications and advantages of the OGDR spectroscopy are discussed.

Sub-Doppler optical-optical double-resonance spectroscopy of BaO: Electronic structure in the 4-eV region

Vibronic levels of BaO with energies in the 4-eV region are observed by cw dye laser optical-optical double-resonance (OODR) spectroscopy. Sub-Doppler, 0.003-cm −1-precision OODR spectra of the C 1Σ +- A 1Σ + (1,0), (2,1), and (3,1) bands are recorded and rotationally analyzed, and numerous perturbations detected in the C 1Σ + state are partially analyzed. Broad bandwidth, 0.2-cm −1-precision, cw dye laser OODR spectra of the C 1Σ +- A 1Σ + (0,0) and (4,2) bands as well as the (3,2), (4,2), (5,3), and (6,3) bands of a new D 1Σ +- A 1Σ + system are obtained and rotationally analyzed. Additional bandheads and several new but incompletely characterized electronic states, B 1Π, E 1Σ +, and c( 3 Π 0) , are also reported. Spectroscopic constants determined for C 1Σ + and D 1Σ + are: C 1 Σ + D 1 Σ + T e ( cm −1 ) 32 872.7 (30) 32 835 (6) ω e ( cm −1 ) 436.0 (8) 403.6 (28) ω e x e ( cm −1 ) 0.90 (16) 3.8 (3) ω e y e ( cm −1 ) 0.041 (10) — B e ( cm −1 ) 0.2397 (6) 0.2197 (20) α e ( cm −1 ) 0.00270 (18) 0.0015 (4) R e ( A ̊ ) 2.2151 (28) 2.314 (10) Quantitites in parentheses are 1σ uncertainties in the last digit.