Ion formation in collisions of Rb(8s) with Rb(5p) and Rb(5s)

Abstract A mass analysis of ions formed in a Rb cell where 5p and 8s states are created has shown that besides associative ionization [Rb(8s) + Rb(5s)], Penning ionization by collision between these two excited atomic species plays a very important role.

Little is known about colhslons involvmg more highly exxlted states, although they may be suspected to play an Important role under many circumstances

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A first estimate of the associative ioruzation rate for reactlons between Rb(9s 2S,,2) and Rb(5p 'P3,') has been obtained by Ch6ret ct al. [S], and some hypotheses on the possible processes creating atomic Ions have been proposed.Owmg to the lack of elpcrimental data, however, our hypotheses could not be corroborated; furtllermore, the densitlcs, deduced from the laser powers, had certainly been ovcresttmated.
Here we have chosen the Rb(8s) state because of the unambiguity ofthe ionic products of the reactlons (2) (3) The apparatus used m tlus c\pcrrment 1~1s been dc-   Assuming that this mcchamsm contributed less than 10% IO Ihe Rbz current.an upper htnit of 2 X 10m9 cm3 s-l is obtained for the rate cocffiacnt.
under study.In contrast to the Rb(9s)-Rb(Ss) reactlon where Rb+ + Rb-Ion-pair formation IS possible, this product channel is energetically excluded for Rb(8s) reacting with Rb(Ss) Our mvestlgation has been performed over a rvlde range of parameters such as rubidium pressure, laser * pcrmancnt address: lnstitut fllr Experimentalphyslk, AbNlufls Atom-und hlolcklllphyak der Uruversltlt Innsbruck, photoiomzatlon by the IWO laser beams, atomic and molecular Ions are formed and are analyzed wtth a quadrupole mass spectrometer.No, N*, N** are the js, 5p and 8s densltles, V the interactton volume, k, and kz the rate coefticicnts of the Penning and associative lomzatlon reactions, at and 07_ tbc photoiomzatlon cross sections at 780 2 and 616.1 nm.The atomic Ion current It is the sum . 4. Ion currents versus pressure.Full and broken curves as calculated with k, = 2 0 X IO-'O cm3 s-l wd kz = 1.3 x 1O-8 cm3 s-l (m and X evperimentat pomts).
photoromrdtion current by the Are I.rser and from the cross-sectron ratros.Under these conditions there IS no doubt about the crcatwn of Rbf ions by collision proccsses The value k, = 2 0 X 10-10 cd s-1 is in agreement wth ,I model of htlhJ~OV and Jcncv [ 141, which predrcts an mcre.tse of k wrth the cffectwe quantum number II* m the vtcimty of II* = 5 corrcspondmg to the 8s and 9s levels of Rb Thus the present data seem to vahddte the model of rcf [ 141 based on the mtcraction of an cxctted clcctron wth the system Rb+ + Rb.It IS not surprrsrng therefore that rate coeffictents for this kind of reactron are of the same order of magnitude 3s rate coefficlcnts for most Ion-molecule reactions with elcctronic transfer which also take place through chargeinduced dipole mteractton (kS from ref. [IS]).The VCIIU~ A-, = I .3X LO-8 03 S-I for reaction(l) bowcver IS much larger tlw the values usually obtained for charge-induced dipole mteracttons, actuaIIy lymg within the range of typical values observed for drssocratrve recombmatron.The transfer of momentum from lhe free electron, whtch IS necessary lo allow for a high recombmatron coefficrent, occurs easily in the case of drssociative recombmation, where two product partrcles exist, but cannot occur u-r the recombmatron of an atomic ton with an electron In the present case however the loosely bound and thus quasi-free electron of the Inghly exctted Rb atom can interact and recombme with the iomc core of the second one, whilst the ejection of the second-exerted electron allows for the balance of momentum 111 the reactive system.this is the so-called ewhange ronwtron process proposed by Olson [ 161.