Collisionally induced ionization of Rubidium atoms in rydberg states

Volume 79, number 3

CHEMICAL

COLLJSIONALLY

T J WHITAKJZR Pacrfic Northnest Received

JNDUCED and B.A Laboratory,

2 April 1980,

III foal

IONIZATION

PHYSICS

LETTERS

OF RUBIDIUM

1 May 1981

ATOMS

IN RYDBERG

STATES

*

BUSHAW Rtcluknd,

Washngtorl

form 29 Januay

99352

L&l

1981

The efficiency of colhsronaJ tomzat1on of excited rubldmm atoms IS demonstxted for Rb*-Kr colhstons Smce the power required to optuxlly saturate the Rydberg levels IS much lower than for photoIonlzntlon, the tcchmque promlsLs to reduce the hwr

Resonance proven useful

requuements

m smgle-atom

lomzatlon spectroscopy several apphcatlons,

m

lute measurement

of excited-state

detectron

[I] (RJS) has mcludlng absopopulations [2,3 j,

colhslonal hne broadenmg [4,5J, studies on dlffuslon of hrghly reactive atoms [6], smgle-atom detectron [ 1] and Isotope separation [7] Among proposed apphcatrons IS the redJctlon of background m lowlevel countmg, usmg tune comcrdent detectlon of daughter atoms by saturated RIS 181 In RIS, a photon at wavelength h, excites an atom from state 1 (usually the ground state) to state 2. Then another photon at wavelength X, or X2 promotes the excited atom from state 2 to the lomzatron contmuum and the resultmg Ion pair IS detected Unfortunately, thus photoronlzatlon step usually has a very low cross sectlon and therefore requires hrgh-power lasers for saturatron. Modlficatlon of RIS has been proposed [8,9] wherem the second laser promotes the excited atom from state 2 to a Rydberg state. The cross sectron for this type of transItIon should be much hrgher than for photoronlzatron The Rydberg atom is then lomzed either by collisIona effects or by the apphcatlon of an electric field. Pulsed field ionizatron studies have been made of Rydberg states of hydrogen [lo], sodrum [ II- 141, and rubldlum [ 151. Several dc field lomzation studies hdW also been made on alkaline metals [ 15-201. Colhstonal lonlzatron of hrghly excited states has recerved some attention m alkaline * Prepared

for the US Department DE-AC06-76lUO 1830.

5l-K

of Energy under Contract

0 009-26

and other resonance

iomzatron

earths [I I ] and Inert tlon of hrghly exerted slons wtth inert gases letter reports a srmple use of thus colllslonal lonlzatton of Rydberg A N,-laser-pumped

elperlments

gases [23]. However, the lomzaalkahne metal atoms by colhhas not been analyzed Thrs techruque which explores the process to produce effictent atoms dye laser (HZnsch

conflgura-

tlon), operatmg

at X, = 780 nm. was used to pump ground-state rubrdtom atoms to the 5 ‘P,lZ state A second oscdlator, pumped by the same N, laser, was scanned m the region between 478 and 5 16 nm to produce an eucrted-state spectrum from above the lomzatron threshold to below the 10 “D level of rub!drum. Ions produced were detected on a tungsten wire charged to = 200 V aoove the body of the stamless steel cell The 1omzat1on slgnal was normahzed with respect to h2, averaged typically over 30 laser pulses and stored on floppy d&s Normahzatlon with respect to h, was not usually requrred because we were strongly saturatmg that transItron Fig. 1 shows the result of such a scan wrth 20 Torr krypton m the cell at a temperature of 70°C The pressure of Rb at the laser mteractron region IS reduced from its normal vapor pressure at this temperature to *:10d6 Torr by trace rrnpurrtres In the cell Amphtude errors m fig. 1 occur ma&y due to random frequency Jitter of the laser and also to a systematic change m the ratio of the Doppler wrdth of mdrvrdual transltlons to the laser band wrdth. This latter effect produces relative errors of ==7% between II = 10 and n = 30 transltlons. Because of these errors, only quahtative

14/8 I /OOOO-0000/3

02 5ilO

North-Holland

Publishmg

Comoanv

Volume

79. number

‘680

CHEMICAL

3

4900 SECONO PHOTON

5000 bhLELEhGTH

PHYSICS

5180

I

CA)

c

Fig 1 lomzat~on spectrum of 5 2Ps/2 rubldlum m 20 Torr of krypton Rb pressure 1s = lo4 Torr The structure observed tn the conttnuum ts for Rydberg + 5 2FIn transluons Prmclpal quantum numbers are given for some of the ‘D levels The (n + 2) *S levels typIcally occur at the nekt peak up ( energy) after n *D

conclustons may be drawn from the spectrum m fig. 1 One obvtous feature IS that the relatrve amphtudes do not fit what one would expect from romzatron due to a sunple thermal colhsron between Rb* and Kr. This nevtdenced by the slow decay of the romzatron srgnal with increasrng wavelength. The 20 D level is located = kT from the ionrzatron threshold. The 10 D level at 5 15 nm 1s at =7kT below the threshold Smce one would expect sample energy exchange vra thermal colhstons to go as exp (-AE/kT), ths imphes a ratio of colhsional ioruzation cross sections, cr,,(20 *D)/oe,(lO

= 400 .

*D) = e-l/eM7

(1)

The quantum defect for D levels of rubrdrum zl 35 [20] Thus, the ratio of optical excrtatron sections IS

IS cross

-3

“lo Therefore

the relative

lonrzatron

1

(2)

srgnal ratio

~202~)

%,(20*D)‘Jop&O*D)

1(1O*D)=

o&O

2D) oopt(102D)

The expenmental ratio tn fig. 1 is 4 off from this model. Correctron width would make thrs discrepancy fore, snnple exchange via thermal out as the principal mechanism of

=40

1 May 1981

LETTERS

(3)

=Z 10 1, a factor of for the Doppler even worse. Therecolhsrons IS ruled ion formation. The

4780

I

4785

4790 479s 4800 4805 SECOND W+OTON VAVELENCTH

4810 (A>

4615

4E

?0

FE 2 Ioruzatlon spectrum of 5 *Psn rubrdmm with < IO4 Torr of buffer gas Note that practtcally all of the tontzatlon signal due to Rydberg levels below the field lonlzatlon lumt has vamshed Thts parttcukir spectrum was made with a rehtlvely large potentl;ll(1600 V) on the countmg wue. Except for the dtsplaced threshold, It IS ldentlcal with other spectra taken at lower V

same type of argument rules out photoromzatron from black-body radratron of the cell Photoromzatron from either Xl or X2 IS ebmmated by consrderatron of fig 2 wluch shows a spectrum stmtlar to fig 1 in which the pressure of Kr has been reduced below 10m6 Torr. The ebmmatron of practrcally all of the srgnal from bound Rydberg states below the field rontzatton lrmrt demonstrates the collrsronai nature of the rontzatron process Decrease of Kr pressure from 20 to =Z1 Torr showed httle or no effect m the romzatron spectrum or the shape of the roruzatton stgnal, rmplymg the cohrsronal process IS two-body The mecharusm for such a process m&t be assocratrve IOIUzatton or perhaps a colhsronal redrstnbutton of energy m the Rydberg levels followed by erther colhstonal ionizatron or photoronrzatton [23]. However, photoronlzatron of the selected state (before colhstonal redtstrrbution of energy) IS not a large effect as IS demonstrated III the spectrum in fig. 2. This spectrum was obtamed wtth our time-gated electrontcs maxrmlzed for detection of photorontzation. The very small signals observed for the bound states shows the small contnbutton of photoionrzatton to the total ronlzatron srgnal. In order to obtam the efticrency of the colbsronal 507

Volume

79, number

CHEMICAL

3

PHYSICS

LETTERS

1 May 1981

ionization implies single atom detection schemes and resonance ionizatron with amplification (RISA) [8] techniques could also benefit from this process.

References [l] G.S. Hurst, M G. Payne, SD. Kramer and J P. Young, Rev. Mod. Phys. 5 1 (1979) 767. and references therem G S. Hurst, M.G. Payne, M.H. Nayfeh, J.P. Judrsh and E.B. Wagner, Phys. Rev. Letters 35 (1975) 82. t31 IMG. Payne, G.S Hurst, M.H. Nayfeh, J.P. Jud1sh.C H Chen. E.B. Wagher and J.P. Young, Phys. Rev. Letters 35 (1975) 1154. [41 M.H. Nayfeh, G S. Hurst, M.G. Payne and J P. Young, Phys. Rev. Letters 39 (1977) 604. PI M.H. Nayfeh, G.S. Hurst, M G. Payne and J P. Young, Phys. Rev. Letters 41 (1978) 302. 161 G.S. Hurst, S.L. Alhnan, M-G. Payne and T J. Whrtaker, Chem. Phys. Letters 60 (1978) 150. [71 B Snavely, Report on the 8th International Quantum Electronics Conference, San Francisco (July 1974). 181 G S. Hurst, SD. Kramer, M.B. Payne and J.P. Young, IEEE Trans. Nucl. Sci. NS-26 (1979) 133. PI L.N. Ivanov and VS. Letokhov, Soviet J. Quantum Electron. 5 (1975) 329. IW AC. Riviere and D.R. Sweetman, Proceedings of the 6th International Conference on Ionization Phenomena in Gases, Paris (1963) p. 105. 1111 T.W. Ducas, M.G. Littman, P.R. Freeman and D. Kleppner, Phys. Rev. Letters 35 (197.5) 366. [=I C. Fabre, M. Gross and S Haroche, Opt. Commun. 13 (1975) 393. El31 J.L. Vialle and H-T. Duong. J. Phys. B12 (1979) 1407. [I41 T.F. Gallagher, L.M. Humphrey, W.E. Cooke, R.M. Hrll and S.A. Edelstein, Phys. Rev. Al6 (1977) 1098. I151 S. Liberman and J. Pinard, Phys Rev. A20 (1979) 507. 1161 S. Feneville, S. Liberman, J. Pinard and A. Taleb, Phys. Rev_ Letters 42 (1979) 1404. [I71 M.G. Littman, MM. Kash and D. Kleppner, Phys. Rev. Letters 41 (1978) 103. 1181 R.R. Freeman, N.P. Economou, G-C. Bjorklund and K-T. Lu, Phys. Rev. Letters 41 (1978) 1463. [I91 W.E. Cooke and T-F. Gallagher, Phys. Rev. Al7 (1978) 1226. PO1 R.V. Ambartzumian, G-1. Bekov, VS. Letokhov and V.I. Mislrin, JETP Letters 21 (1975) 279. 1211 J.A. Armstrong, P. Esherick and J J. Wynne, Phys Rev. Al.5 (1977) 180. Wl K. Radler and J. Berkowitz, J. Chem. Phys 70 (1979) 221. 1231 F. Gorinand, P.R. Fournier and J. Berlande, Phys. Rev. A15 (1977) 2212. [2]

SECOND

PHOTON

WAVELENGTH

CA)

Fig. 3. Optically saturated spectrum of Rb. In this spectrum the beam from the second laser is focused so that the photoionization signal is no longer linear with laser intensity. Note that even this close to saturation, more ions are being produced from several of the Rydberg levels.

ionization process, we have taken a spectrum in which the signal produced in the photoionization continuum is nearly saturated (fig_ 3). The spectrum was produced by focusing the X, beam into the cell. The beam intensity is such that when AZ intensity is decreased by a factor of 4, the photoionization signal is reduced by about a factor of 2. Note that even this far up the saturation curve, more ions are being produced when ?Q is tuned to the 20 2D level, implying near unity probability for this level to be ionized. Thus, the cross section for the ionization process(es) is much larger than that for collisional deactivation, and at pressures of above 1 Torr, occurs more rapidly than spontaneous emission or non-radiative decay mechanisms. Inspection of fig. 1 shows a factor of 10 is gamed in ionization signal by exciting to the 20 2D Rydberg line instead of using photoionization. This comparison is based on photoionization to just above the ionization threshold (cross sections for photoionization typically decrease rapidly above threshold)_ Based on the improvement in excitation by using a narrower band-width laser, we conservatively estimate that exciting the proper Rydberg line could improve ionization signals by at least two orders of magnitude for many RIS experiments. The high efficiency of the

508