Chin-Chun Tsai

Proposed modification of the criterion for the region of validity of the inverse-power expansion in diatomic long-range potentials

Abstract A modified internuclear distance criterion, R LR- m , as the lower bound for the region of validity of the inverse-power expansion of the diatomic long-range potential is proposed. This new criterion takes into account the spatial orientation of the atomic orbitals while retaining the simplicity of the traditional Le Roy radius, R LR for the interaction of S state atoms. Recent experimental and theoretical results for various excited states in Na 2 suggest that this proposed R LR- m is an appropriate generalization of R LR .

Observation of the 4 3Σ+g, 3 3Πg, 2 3Δg, and b 3Πu states of 39K2 by perturbation facilitated optical–optical double resonance spectroscopy

The 4 3Σ+g, 3 3Πg, and 2 3Δg states of K2 have been studied by perturbation facilitated optical–optical double resonance spectroscopy for the first time. Molecular constants of those states are derived and compared with theoretical calculations. Varying the trial vibrational quantum numbers of these upper states, absolute vibrational quantum number assignments are obtained by comparison between the observed resolved fluorescence intensities and the calculated Franck–Condon factors. The Rydberg–Klein–Rees potential curves are constructed and compared with theoretical calculations. Finally, the observation of the lowest ‘‘dark’’ vibrational levels of the b 3Πu state by resolved fluorescence from the 2 3Δg state confirmed the absolute vibrational numbering of this state and allowed refinement of the b 3Πu molecular constants.

Determination of the long-range potential and dissociation energy of the 1 3Δg state of Na2

The 1 3Δg state of Na2 has been studied extensively by both filtered fluorescence and ionization detection and analyzed by both Dunham‐type expansion and near‐dissociation expansion (NDE) models in the analysis. Our observations have covered 99.998% of the potential well depth with the outermost Rydberg–Klein–Rees (RKR) turning point at 28.02 A. NDE analysis gives Te=28 032.468 (±0.021) cm−1, De=7162.436 (±0.021) cm−1, and Re=3.463 81 (±0.000 28) A. Significant long‐range behavior in the near dissociation levels has been observed. Fitting of the RKR turning points gives the long‐range coefficients C5=1.388 (±0.031)×106 cm−1 A5 and C6=0.4008 (±0.0046)×108 cm−1 A6. These newly observed results show reasonable agreement with recent theoretical calculations.

Shielded cylindrical space‐charge‐limited diode ionization detector

A shielded cylindrical space‐charge‐limited diode ionization detector with high sensitivity and high resolution has been constructed to detect ionization in metal vapors, e.g., multiphoton ionization in the sodium atom and collisional associative ionization in the Rydberg states of the sodium dimer. The diode design and construction and measurements of its characteristics are described. Using this diode detector, a wide variety of high resolution spectroscopy can be obtained using techniques such as two‐photon excitation, optical–optical double resonance, perturbation‐facilitated optical–optical double resonance, and all‐optical triple resonance.

Observation of Na2 Rydberg states and autoionization resonances by high resolution all-optical triple resonance spectroscopy

Abstract Na2 Rydberg states have been observed for 10 ⩽ n ⩽ 119 and 0 ⩽ J ⩽ 9 using high resolution all-optical triple resonance spectroscopy and an ultrasensitive ionization detector. This represents the first example of high resolution triply resonant photoionization with cw lasers in any molecule. An improved ionization potential of Na2 was estimated by extrapolation to be IP = 39478.75 ± 0.04 cm−1, corresponding to an improved molecular ion dissociation energy De = 7973.83 ± 0.12 cm−1. Four series, n p 1 ∑ u + , n p 1 Π u , n f 1 ∑ u + and n f 1 Π u , were found and rovibrational interactions, l uncoupling, stroboscopic effects and Fano autoionization profiles were observed in the spectra.

Optical–optical double resonance spectroscopy of the 4 1Σ+g ‘‘shelf’’ state of Na2 using an ultrasensitive ionization detector

The 4 1Σ+g ‘‘shelf’’ state of Na2 was extensively investigated using optical–optical double resonance spectroscopy and an ultrasensitive ionization detector. A total of 179 vibrational levels‐(0≤v≤127 are from previous work by using filter‐photomultiplier tube fluorescence detection) were observed in this state, which covered more than 99.8% of the well depth. The Rydberg–Klein–Rees (RKR) potential energy curve was constructed with maximum outer turning point R178+ = 17.60 A. The comparison of the long range portion of the RKR potential energy curve and the potential curve calculated from V(R)=ΣnCn/Rn is discussed. Avoided crossings around the shelf with the 3 1Σ+g state and near dissociation with the 5 1Σ+g state are also discussed.

The 3 1Σ+g ‘‘shelf’’ state of Na2

The 3 1Σ+g state of Na2 has been extensively studied using an optical–optical double resonance technique and a shielded cylindrical space‐charge‐limited ionization detector. A wide range of vibrational quantum numbers 0≤v≤141 was observed which covered over 99.9% of the potential well depth. A ‘‘shelf’’ on the potential energy curve was found near v=23. A hybrid potential was constructed based on the Rydberg–Klein–Rees (RKR) potential curve and a local inverse perturbation approach (IPA) for the shelf region. For this potential, there is a shallow second local minimum ∼5 cm−1 in depth at the shelf region. The long range energy also compares well with long range theory.

First observation of the quasibound levels and tunneling line broadening in the 3 1Πg state of Na2 using an ultrasensitive ionization detector

All quasibound vibrational energy levels v=27 and 28 of the Na2 3 1Πg state were accessed by using optical–optical double resonance and detected with a shielded ionization detector. The line broadening due to the tunneling effect was observed to be larger than 1 cm−1 at the rovibrational level v=28, J=31. The potential curve has been determined by the Rydberg–Klein–Rees method and modified using Jeung’s calculation for the large distance barrier and comparison of the calculated and the measured line widths. The barrier height was found to be ∼126 cm−1 above its asymptotic limit (3s+4p) at 5.98 A. There also exists an unobserved shallow outer well with depth ∼35 cm−1 at 9.5 A according to our modification of Jeung’s calculated potential. One way to possibly probe this outer well is also discussed.

Observation of L uncoupling in the 5Δg1 Rydberg state of Na2

The phenomenon of electronic orbital angular momentum L uncoupled from its internuclear axis has been observed in the sodium dimer using high-resolution cw optical-optical double-resonance spectroscopy. When L uncoupling occurs, the degeneracy of Λ doubling is removed. In our experiment, the intermediate BΠu1 state of Na2 is excited from the thermally populated ground XΣg+1 state by a single-line Ar+ laser. Then, a single-mode dye laser is used to probe the Rydberg states from the intermediate state. The signals are detected by monitoring the UV fluorescence from the triplet gerade states back to the aΣu+3 state via collision energy transfer. Under our experimental resolution, the splitting of Λ doubling in the 5Δg1 state of Na2 can be measured. A total of 136 rovibronic levels with e∕f parities have been assigned to the 5Δg1 state. The Λ-splitting constants deduced from these data are q0=0.376(90)×10−4cm−1, qv=0.114(6)×10−4cm−1, and μ=0.76(33)×10−8cm−1. In general, the Λ splitting of the Δ states is cons...

Time-independent and time-dependent photoassociation of spin-polarized rubidium

We extract information about collisions of ultra-cold ground-state rubidium atoms from observations of a g-wave shape resonance in the 85Rb + 85Rb system via time-independent and time-dependent photoassociation. The shape resonance arises from a quasi-bound state inside a centrifugal barrier that enhances the excitation to the bound electronically excited state by the photoassociation laser in the time-independent experiment. The shape resonance is sufficiently long-lived that its build-up through the barrier can be observed by first depleting it via a photoassociation laser pulse and then measuring the rate of photoassociation by a second laser pulse with a variable delay time. A combined method of analysis of the time-independent and time- dependent experiments is presented. We discuss the spectroscopy of states of two particles with spin trapped inside a centrifugal barrier, interacting via direct and indirect spin-spin interactions.