Extensive new data on the odd-parity, single-colour, two-photon optogalvanic spectra of argon in the visible region is reported using an excimer-pumped dye laser in conjunction with an argon discharge cell. The experimental technique enables us to record a pure two-photon spectrum in a region which is dominated by one-photon transitions. A large number of two-photon transitions, originating from the two metastable states 3p 5 4s[3/2] 2 and 3p 5 4s [1/2] 0 to the excited states of configurations 3p 5 ns (J = 0, 2) and 3p 5 nd (J = 0-4) of argon, has been recorded and arranged into 10 Rydberg series. The interchannel interactions among the 3p 5 ( 2 P 1/2 )nd and the 3p 5 ( 2 P 3/2 )ns, nd J = 2 channels and the 3p 5 ( 2 P 1/2 )nd and the 3p 5 ( 2 P 3/2 nd J = 3 channels have been studied using the multichannel quantum defect theory (MQDT). In addition, higher members of the two-photon parity forbidden 3p 5 np series have also been detected.

We report new data on the odd-parity bound and autoionized Rydberg series of krypton using single-colour two-photon laser-optogalvanic spectroscopy. These series were excited from the and metastable levels. A total of 10 odd-parity , , , , , and unresolved (or ) Rydberg series are observed. The , , (or ) and Rydberg series have been observed up to n = 49, 44, 39 and 36 respectively. The interchannel interaction among the overlapping Rydberg series has been studied using the multichannel quantum-defect theory. In addition, the two-photon parity-forbidden Rydberg series excited from the level is also observed.

We report new data on the bound odd-parity Rydberg series of xenon using single-colour two-photon laser optogalvanic spectroscopy. These series are excited from the 5p 5 6s 3 2 2 metastable level. A total of eight odd-parity 5p 5 nd 1 2 0 (12 n 21), 5p 5 nd 1

We report extensive new data on the single-colour two-photon optogalvanic spectra of neon in the visible region. The experimental method enables us to record a pure two-photon spectrum in a region which is full of one-photon transitions. A large number of two-photon transitions originating from two metastable 3s(3/2)2, 3s'( 1/2 )0 states and one non-metastable 3s(3/2)I state of the 2p53s configuration of neon have been recorded and arranged into 19 Rydberg series. The interchannel interactions among the overlapping Rydberg series have been studied using the multichannel quantum defect theory.

We have discovered a novel, spatially propagating wave front which is generated by laser pumping of an optically thick medium to a state of complete transparency. The wave front consists of a moving band of fluorescent light which marks the location of atoms which are being pumped. The wave-front velocity v, the photon flux /Q, the atomic number density iV, and the number n of photons absorbed per atom are related by f =/o x(JVn)"^