Lifetimes and transition moment of the A-X band system of Bi 2

Abstract

Lifetimes of excited levels (υ′, J′) in the A state of Bi 2 have been measured using a N 2 pumped dye laser amplifier system and optical pulses generated from single mode cw lasers by the scanning mirror technique. The exponential decay has been monitored by detecting the fluorescence of a single P or R line. The measured lifetimes of the investigated levels (υ′ = 1–34) range between 50 and 600 ns. The data indicate that vibrational levels υ′ ⩾ 27 are strongly predissociated. The determined foreign gas quenching cross sections σ(Bi 2*-Ar) range between 50 Å 2 and 100 Å 2 depending on the specific level (υ, J′). For the levels υ′ = 10, J′= 160 and υ′ = 17, J′ = 255 the self quenching cross sections have been determined to σ-(Bi 2*-Bi 2) =280±30 Å 2 and 320±40 Å 2, respectively. From the variation of the lifetimes with υ′ and from relative intensity data we found, that the electronic transition moment does not vary between the internuclear distances 2.6 and 3.3 Å. The value of the transition moment of the A-X band system has been determined to R a(r)=1.4±0.4 D. In this paper detailed measurements of life-times of the A-X band system of Bi 2 are reported. Owing to the very dense A-X absorption spectrum, the lifetimes were measured with a narrow band pulsed dye laser system (Δυ ≈ 2–3 GHz; fwhm) and with short light pulses generated from single mode cw lasers (Ar +, Kr +) by the scanning mirror technique [9]. With both techniques we obtained corresponding results. In addition to the narrow band excitation, a narrow band detection was employed by monitoring the fluorescence decay via a single P or R transition. The measured lifetimes vary between 50 and 600 ns. The whole set of results is given in table 1. The most important features are the slowly increasing decay rates 1 τ eff for levels above υ′=20 and the high decay rates ( 1 τ eff ≈ 20 × 10 6 s −1) of levels υ ⩾ 27. Simultaneously with the shortening of the lifetimes, we noticed an important decrease of fluorescence signal. A likely explanation of the observed phenomenon is a predissociation of the A state. The dependence of the lifetime τ eff upon the rotational quantum number J was investigated for selected vibrational levels υ′. We observed in most cases a decrease of τ eff with increasing J values. From the variation of the lifetimes with the Ar gas pressure (fig. 5) the foreign gas quenching cross sections σ(Bi 2*-Ar) were determined, which range between 50 and 100 Å 2 depending on the specific excited level (υ′, J′). Our measurements also show, that for pressures p(Ar) ⩾ 1 Torr multiple collisions play a role in repopulating the originally excited levels. For the upper levels υ′ = 10, J′ = 160, and υ′ = 16, J′ = 198 we measured the lifetimes as a function of the molecular partial pressure p(Bi 2) at constant foreign gas pressure (fig. 6). The slope of the linear extrapolation-which is correct only up to p(Bi 2) ≈ 0.1 Torr — yields the self-quenching cross section σ(Bi 2*-Bi 2), which is for both investigated levels ≈300 Å 2. A comparison (section 5) with the only previous lifetime measurements of the A-X system of Bi 2 reported by Blondeau et al. [10] shows agreement with the gross features, but differences with individual results. Since it is very difficult to obtain accurate collision free lifetimes τ 0 of the isolated Bi 2 molecule from gas cell measurements, we currently prepare experiments to measure lifetimes with a Bi 2 molecular beam. From our lifetime data, we determined the absolute value of the electronic transition moment of the A-X band system of Bi 2 to R e( r) = 1.4±0.4 D. The value of R e( r) was found to be independent of the internuclear distance r in the range of 2.6 to 3.3 Å from both the variation of lifetime with υ′ and from relative intensity measurements of several fluorescence series.