M Rafiq

Laser isotope separation of lithium by two-step photoionization

Lithium isotope separation has been achieved employing the two-step photoionization technique along with a narrow band dye laser in conjunction with a time of flight mass spectrometer. The demonstrated method yields a high degree of selectivity by tuning the dye laser at the resonance levels of Li6 and Li7. It is inferred that the concentration of the natural abundance of the Li6 isotope gets enhanced up to over 47% as the exciter dye laser is tuned to the P1∕22 of Li6 even if the linewidth of the exciter laser is not sufficiently narrow to excite the isotopic level. It is also noticed that the much higher energy density of the exciter laser limits the resolution of the fine structure levels of the lithium isotopes that leads to a loss in the enrichment of Li6 due to the power-broadening effect. Measurements of the photoionization cross section of the lithium isotopes from the 2pP1∕2,3∕22, excited states for Li6 and Li7 and the corresponding number densities are reported.

Simultaneous measurements of photoionization cross-sections of lithium isotopes from 3p 2P1/2, 3/2

We present the first measurements of the photoionization cross-section of the 3p 2 P1/2, 3/2 excited states for both the lithium isotopes Li 6 and Li 7 in the region close to the Li + 1s 21 S threshold (from 0 to 3 eV) using the two-step photoionization and the saturation technique. Narrow linewidth dye lasers have been used in combination with a locally developed atomic beam-TOF mass spectrometer to measure the photoion signals as a function of energy density of the ionizing laser. It is observed that the cross-section is maximum near the ionization threshold and decreases monotonically as the excess photon energy of the ionizing laser is increased. The deduced number densities of the isotopes are Li 6 (1.2 × 10 8 atoms cm −3 ) and Li 7 (1.2 × 10 9 atoms cm −3 ), which are found to be close to their natural abundance ratio within the experimental error. Our experimental results of the measurement of the cross-section are also in good agreement with earlier theoretical work. (Some figures in this article are in colour only in the electronic version)

Absolute photoionization cross section from the 6s6p 1,3P1 excited states of barium

We present photoionization cross section measurements from the 6s6p 1 P1 and 3 P1 excited states of barium at and above the first ionization threshold. The experiments have been performed using a thermionic diode ion detector in a space charge limited mode in conjunction with a Nd:YAG laser system and the saturation technique to determine the photoionization cross sections. The absolute values of the photoionization cross section from the 6s6p 1 P1 and 3 P1 excited states at the first ionization threshold have been determined as 90 ± 14 Mb and 102 ± 15 Mb, respectively. The measured values of the photoionization cross section from the 6s6p 1 P1 excited state are compared with available experimental and theoretical work, while the absolute photoionization cross sections from the 6s6p 3 P1 excited state are reported for the first time. (Some figures in this article are in colour only in the electronic version)

Measurement of photoionization cross section from the 3s3p 1P1 excited state of magnesium

The photoionization cross section from the 3s3p 1P1 excited state has been measured in the energy region from the first ionization threshold up to 1.4 eV excess energy using a two-step photoionization and saturated ionization technique in conjunction with an atomic beam source and a time-of-flight mass spectrometer that enables the separation of the three stable isotopes of magnesium on the time axis. The absolute value of the photoionization cross sections from the 3s3p 1P1 excited state near the 3s ionization threshold has been measured as 90 ± 16 Mb (at 354.5 nm ionizing wavelength) for the dominating isotope (24Mg) whereas the value at the peak of the 3p2 1S0 auto-ionizing resonance has been determined as 785 ± 141 Mb. The present experimentally measured photoionization cross sections are compared with the existing experimental and theoretical work, showing excellent agreement.

Photoexcitation study of the 4s 2 S 1/2 state of atomic sodium

We report the first measurements of oscillator strengths for 4s 2 S1/2 → np 2 P1/2,3/2 (19 n 57) Rydberg transitions in atomic sodium using a thermionic diode ion detector in conjunction with an Nd:YAG pumped dye laser system. The f-values have been calibrated with the measured photoionization cross-section from the 4s 2 S1/2 excited state at the first ionization threshold as 0.65(0.10) Mb. The binding energy of the 4s 2 S1/2 level is determined as 15 709.444(8) cm −1 by employing the Rydberg relation to the observed np 2 P1/2,3/2 Rydberg series. Addition of the binding energy to the known energy of the 4s 2 S1/2 level yields the first ionization potential of sodium as 41 449.44(1) cm −1 , which is in excellent agreement with the recognized value. (Some figures in this article are in colour only in the electronic version)

Experimental studies of the oscillator strengths of the 6s6p 1,3P1 → 6snd 1,3D2 Rydberg transitions in barium

We report new measurements for the oscillator strengths of the 6s6p 1P1 → 6snd 1D2 and 6s6p 3P1 → 6snd 3D2 Rydberg transitions of barium using a thermionic diode ion detector in conjunction with a Nd:YAG pumped dye laser system. The f-values have been calibrated with the absolute photoionization cross-section measured at the first ionization threshold from the 6s6p 1P1 and 6s6p 3P1 excited states employing the saturation technique. An excellent agreement is found for the 6s6p 1P1 → 6snd 1D2 Rydberg transitions with the earlier reported work. The data on the oscillator strength of the 6s6p 3P1 → 6snd 3D2 Rydberg transitions are reported for the first time. The oscillator strength densities in the continuum corresponding to the 6s6p 3P1 excited state have also been determined and a smooth merging of the discrete f-values into the oscillator strength densities is observed across the ionization threshold.

Multi-photon excitation spectra of the 3snℓ (ℓ = 0, 1, 2 and 3) Rydberg states of magnesium

New experimental data on the highly excited l = 0, 1, 2 and 3 Rydberg states of magnesium have been acquired using two-photon and two-step laser excitation technique in conjunction with a thermionic diode ion detector. The new observations include even parity 3sns 1S0 (8 ≤ n ≤ 24) and 3snd 1D2 (7 ≤ n ≤ 62) Rydberg states approached directly from the 3s2 1S0 ground state via two-photon excitation, and the odd parity 3snp 1P1 (20 ≤ n ≤ 61) and 3snf 1F3 (14 ≤ n ≤ 66) Rydberg states accessed by the two-step excitation process via 3s4s 1S0 and 3s3d 1D2 intermediate states. The Rydberg relation fit to the new data of the np 1P1 and nf 1F3 series yields the binding energies of the 3s4s 1S0 and 3s3d 1D2 levels as 18u2009167.702 cm−1 and 15u2009267.972 cm−1, respectively. By adding the binding energies to the corresponding energies of the aforementioned levels, a precise value of the first ionization potential of magnesium is determined as 61u2009671.04 ± 0.04 cm−1. Using this ionization potential value, the quantum defects for the ns 1S0, np 1P1, nd 1D2 and nf 1F3 Rydberg series have been determined as 1.526(2), 1.046(2), 0.602(2) and 0.049(2) cm−1 respectively.