In the present work we have studied the time-resolved optogalvanic signals in the spectra of inert gases. The effects on the optogalvanic signals by scanning a dye laser across the dipole transitions in a DC discharge plasma have been investigated. Time-resolved spectra are obtained at fixed wavelengths of the dye laser, resonantly tuned to optically allowed transitions. The temporal evolutions of the signals are registered on a storage oscilloscope. In the first set of experiments we have selected three transitions from the 3s[l/2]2 metastable state of neon corresponding to the Î”J = Î”K= 0, Â± 1 dipole selection rules to investigate the dominant physical processes responsible for the optogalvanic signals. Based on a theoretical model, the observed signals are fitted to an expression,
S(t)= a [e-bt- e-t/ Ï„ ]+ c [e-dt- e-t/ Ï„ ] 1-bÏ„ 1-d Ï„
A non-linear least squares programme has been used to fit the above equation to obtain the parameters that determine the amplitudes and decay rates of the investigated transitions. The change in the signal amplitude as a function of discharge current has been studied. In addition, the electron collisional ionization rate parameter ratios have been determined for the transitions corresponding to the dipole selection rules Î”J = Î”K = -1,. Î”J = Î”K = 1 and Î”J = Î”K = 0, as 1.63,1.75 and 1.00 respectively. The effective lifetimes of the upper levels involved in the aforementioned transitions are also calculated as 62.50 Âµs, 31.25 Âµs and 12.85 Âµs respectively.
We have further extended the work to study the temporal evolution and variation of the laser optogalvanic signals in the spectra of inert gases using commercial hollow cathode lamps. The behavior of the optogalvanic signals for the transitions following the Î”J = Î”K = 0, Â±1 dipole selection rules in the jcK-coupling scheme namely mp5(m + l)p[5/2]3, mps(m +l)p'[3/2]2, mps(m + l)p[3/2]2, mp5(m + l)p'[l/2]1 and mp5(m + l)p[l/2]1 coupled with mp5(m + l)s[3/2]2 (m = 2,3,4 and 5 for neon, argon, krypton and xenon, respectively) metastable states have been investigated. An empirical relation is formulated:
S (t) = Smin + A exp( -Î± [I -10]), which satisfactorily describes the shift of the optogalvanic signal and the change in the population of the upper level as a function of the discharge current.
In a separate set of experiments, we have determined the oscillator strengths of the 2p5 (2P1/2)nd J =2, 3 auto-ionizing resonances in neon using a DC discharge plasma. The highly excited odd parity states are approached using two-step laser excitation via three intermediate states which are populated from the 2p53s [l/2]2 metastable state, that gets populated by the discharge in the hollow cathode lamp. The f-values have been determined for the nd'[3/2]2. nd'[5/2]2 and nd'[5/2]3 series following the = Î”K =Î”J= +Î”â„“ selection rules. Employing the saturation technique the photoionization cross section at the 2p5 2P1/2 ionization threshold is determined as 5.5(6) Mb and consequently we have extracted the f-values of the auto-ionizing resonances.
In addition, plasma parameters have been determined in the neon hollow cathode discharge lamp. The measurements have been carried out to determine the electron temperature using the intensity ratio method and spatial profile of the electron density using the Stark broadening method. The electron density is measured as a function of discharge current, while the electron temperature 2500(Â± 10 %) K is obtained at a fixed discharge current.