Optical resonance detection by field ionization of Rydberg state in colinear laser spectroscopy

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Optical resonance detection by field ionization of Rydberg state in colinear laser spectroscopy

N. Bendali, H.T. Duong, P. Juncar, J.K.P. Lee, J.M. Saint-Jalm, J.L. Vialle

To cite this version:

N. Bendali, H.T. Duong, P. Juncar, J.K.P. Lee, J.M. Saint-Jalm, et al.. Optical resonance detection

by field ionization of Rydberg state in colinear laser spectroscopy. Journal de Physique, 1986, 47 (7),

pp.1167-1173. �10.1051/jphys:019860047070116700�. �jpa-00210306�

Optical ^{resonance detection} by field ionization of Rydberg ^state in colinear laser spectroscopy

N. Bendali (1), ^{H. T.} Duong (2), ^{P. Juncar} (3), J. K. P. Lee (4), ^{J. M.} Saint-Jalm and J. L. Vialle (5)

Laboratoire Aimé Cotton (*), ^CNRS II, Campus d’Orsay,

Bâtiment 505, ⁹¹⁴⁰⁵ Orsay Cedex, ^France

(Reçu le 11 octobre 1985, révisé le 24 février ^1986, accepté ^{le 6 mars} 1986)

Résumé. 2014 Deux nouvelles méthodes efficaces, ^{et non} optiques, de détection de résonance optique ^en spectroscopie

colinéaire sont décrites. La première ^{méthode utilise} l’ionisation par champ d’un état de Rydberg peuplé ^{à l’aide}

de deux lasers : le premier ^{est monomode et} interagit colinéairement avec le jet ^d’atomes rapides ^{et le second est}

multimode et interagit perpendiculairement. ^En détectant, ^{à la} résonance, ^{les ions} produits ^{à la} place ^des photons

de fluorescence, ^{on obtient une} efficacité de 1,1 ^{x 10-5} qui ^est comparable ^{à la} limite de sensibilité habituellement atteinte par la méthode de fluorescence induite par laser. Une deuxième expérience ^similaire utilise deux lasers à colorant monomodes tous deux interagissant colinéairement avec le faisceau d’atomes rapides. ^Par comparaison

avec la première méthode, ^{on obtient un} accroissement de la sensibilité de deux ordres de grandeur (correspondant

à une efficacité de 1,2 ^x 10-3). L’application ^{de ces} méthodes à l’étude d’isotopes ^{de courte durée de} vie, produits

en ligne, ^{est examinée.}

Abstract

New efficient non-optical detection methods of optical resonances in colinear laser spectroscopy

are described. A first method uses the field ionization of a Rydberg ^state populated by ^{means of two lasers : one}

is single frequency and interacts colinearly with the fast atomic beam, ^{the other one} is multimode and interacts

perpendicularly. By detecting, at resonance, the produced ions instead of the fluorescence photons, ^{an overall} efficiency ^{of 1.1 x 10-5 has been} obtained, ^{which is} comparable ^{to the usual} sensitivity limit reached by ^{the laser}

induced fluorescence method. A second similar experiment ^{uses two} single ^mode dye ^{lasers both} interacting colinearly with the fast atomic beam. Compared ^{to the first} technique ^an increase of the sensitivity by ^{two orders}

of magnitude has been obtained (corresponding ^{to an} efficiency ^{of 1.2 x 10-} 3). Application ^{to the} study ^{of on line} produced short-lived isotopes ^{is discussed.}

Classification

Physics ^Abstracts

35.80

1. Introduction.

The collinear laser spectroscopy method, first intro- duced independently by Kaufman and Wing et ^al.

[1, 2], ^{has been} widely ^{used in} high resolution laser spectroscopy experiments ^{on the on} line, produces

short live isotopes [3]. ^{Thanks to the} longitudinal velocity bunching, the residual Doppler ^{width can} approach the natural line width, therefore, ^{in colinear}

Present addresses :

(1) Universite d’Alger, Alg6rie (Z) C.E.R.N. Isolde Geneve, ^Suisse

(3) ^{Institut National de} Metrologie, ^CNAM Paris, ^France (4) ^{Mc Gill} University Montreal, ^Canada

(5) Universite de Lyon ^I, Laboratoire de spectrometrie ionique ^et mol6culaire, Villeurbanne, ^France

(*) Laboratoire associe a F Universite Paris Sud.

excitation, almost all atoms of the beam interact

simultaneously with the laser light. This efficient ^use of the available rare atoms permits ^the study ^of isotopes far from the stability valley, generally pro- duced in a small quantity; ^this region ^{is of} physical

interest owing ^to nuclear deformations predicted by theory [4].

Up ^{to now,} optical resonances are detected by

observation of the laser induced fluorescence. Link tations of the sensitivity ^of this detection method are

mainly ^{due to the} quantum efficiency ^{of the} detector, ^to

the finite solide angle ^for light collection, ^{to the} optical pumping effects and also ^to the background ^{noise due}

to the laser stray light. ^{The best} sensitivity quoted ⁱⁿ

the literature corresponds ^to the detection of optical

resonances with an atomic beam flux of a few 10’

atomss [5].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019860047070116700

1168

The _purpose of this _paper is to describe an alter- native method of non optical detection of optical

resonances in colinear laser spectroscopy. ^{In this} method, ^the resonant atoms in a first laser field are

excited to a Rydberg ^state by ^the light ^{of a second}

laser. Once in this Rydberg ^state, they ^{are ionized} by

an electrostatic field. One detects, then, either the emitted electrons or the formed ions using ^{an electron} multiplier. ^{Such a} detection scheme has already ^been

used in experiments ^on thermal atomic beams _perpen-

dicularly lighted by ^a laser beam [6], ^{and on fast beams} colinearly lighted [7].

Figure ¹ represents ^the stepwise excitation scheme.

The beam of a laser, tunable around frequency v 1, interacts colinearly with the fast atomic beam

allowing high resolution studies of the hyperfine

structure of the A - B transition and of the isotope ^shift.

The second step (transition ^{B -} C) ^{does not need a} single ^mode excitation. On the contrary, ^{it is} interesting

to keep the second laser frequency v2 ^{tuned to reso-}

nance with the transition B - C while the frequency via

of the first laser ^scans the hyperfine ^{structure of the}

line corresponding ^{to the A - B} transition. The band- width of this second laser light ^{must be therefore} sufficiently ^{broad to cover the structure of the B level}

for all different available isotopes. ^For the second step of excitation either a CW or a powerful pulsed ^laser

could be used. In the later case a dye ^laser pumped by ^a

copper vapour laser [8] (L - 5 x 10- 8 s, f =

5 ^x 103 Hz) ^would give ^an efficiency ^{up to 2.5 x 10-4}

since the efficiency is limited by ^the duty cycle; pulsed

laser with lower repetition ^{rate, as in the case of excimer} laser, ^would degrade ^the efficiency. ^{As we shall see in}

the next section, ^{we have} preferred, ^for this, ^{a multi-}

mode CW dye ^laser interacting ^{with the atomic beam}

several times and at different angles ^{around a mean}

direction perpendicular ^to the atomic beam. Then in the third step ^the atoms, prepared ^{in the,} Rydberg

state C, ^are ionized with complete efficiency by applying ^an electrostatic field E a few cm downstream the laser interaction region. ^{Because ions} (or electrons)

are only produced ^{when the} frequency of the first laser is exactly ^{tuned to a} particular resonance, the signal

delivered by the electron multiplier ^{appears on a zero} background in the ideal case. The sensitivity ^{of this} method, ⁱⁿ comparison with the method of detection

by the laser induced fluorescence will therefore mainly depend upon the efficiency of the second step ^{of the} excitation. We shall see, in section 3, that, ^{with the} power-limited ^laser light used in the experiment, ^this efficiency ^{can be estimated to a few time 10- 3.}

2. Experimental set-up.

The experiment has been done on a fast neutral Na beam. Figure ^{2 shows the} experimental set-up. ^The part ahead the interaction and detection chamber has already been described in previous papers [9, 10].

Briefly, sodium ions are accelerated _up to 5 keV and form a mono kinetic fast ion beam. This beam is

Fig. ^1.

Excitation scheme : in the first step ^the ground (or metastable) ^{state A is excited} towards the B state by ^the light

of a single ^{mode CW tunable} dye ^{laser of} frequency vl. In the second step (transition B-C) ^the light ^is provided by ^a

multimode CW dye ^laser (frequency v,). In the third step, the Rydberg ^{atoms are ionized} by ^{means of an} electrostatic field E.

deflected in order to be set colinear with the light ^{of a}

CW single ^mode dye laser. Fast sodium ions are

neutralized by charge-exchange collisions in a 30 cm

cell of recycled ^sodium vapour [11]. ^The temperature ⁰ of the centre of the cell is set at 250 OC and kept

constant. Approximately ⁵⁰ % ^{of the} incoming ^Na

ions are neutralized into the ground ^state (2S1/2) ^of

the neutral Na atom and form a fast monokinetic neutral beam. The laser radiation of frequency v 1 is

provided by ^a single mode commercial dye ^laser (Coherent ^CR 599/21) running ^{with rhodamine 6 G}

and frequency controlled by ^a sigmameter [12]. ^The

laser frequency is first tuned to resonance with the

hyperfine component ^{F =} 2 ---> F’ ⁼ 3 of the D2 ^line (589 nm) for which there is no optical pumping ^effect.

A small periodic ramp voltage ^{of a} few volts is then

applied ^{to the} charge exchange cell. In this _way, the laser frequency ^seen by ^{the atoms can be} easily ^and rapidly Doppler ^{scanned over} the width of the selected component by varying the ionic beam, ^{and hence}

atomic beam, velocity. ^The fluorescence signal ^emitted

at resonance is detected by ^a photomultiplier. ^{A few cm}

downstream the charge exchange cell, ^{the beam}

crosses a region where it interacts several times with the quasi-perpendicular ^{beam of a} second laser.

This laser is also a Coherent model 599/21 ^but working

in a multimode configuration (without the internal-

cavity-assembly) ^with only ^the birefringent ^filter (Lyot

Fig. ^2.

Experimental ^set up : P.M. photomultiplier; E.M. electron multiplier type ^{SEV 317} (from BALZERS); F.A. &

D. fast amplifier (ELSCINT ^model TFA.N.I) ^and discriminator (ORTEC ^model 436); multichannel analyser ^NUMELEC

INTERZOOM.

Filter) ^as dispersive element. It runs with stilben III and is pumped with 1.8 U.V. watts of a krypton ^ion

laser (Coherent ^{CR 3000} K). ^{A small} speed ^motor (2 turns/hour) ^{fixed on} the micrometer screw of the

Lyot ^Filter permits ^to slowly ^and precisely ^{scan the}

mean frequency v2 of the laser. Two Rydberg ^levels

can be reached with our system :

-

the lOd level at 427.6 nm above the 2P3/2 ^excited

level

-

and the 11 d level at 421.6 nm.

The _power emitted by ^{the laser at these} wavelengths

is about 100 mW. The bandwidth is approximately equal ^{to 0.3 cm-1. As shown} in detail in figure ^{3 two} high quality ^and high reflectivity ^{12 cm} long mirrors, approximately parallel ^{to the} fast beam axis and located 50 cm apart ^on both sides of it, reflect the beam of the second laser several times. This system ^{is called}

« multipass systems. ^A slight angle ^of typically p = 2.5 ^{x 10-4} rad introduced between the two mirrors permits ^to reflect the beam at different angles (as mentioned in the introduction). The direction of the incoming beam and the direction perpendicular

to the fast atomic beam differ by ^an angle 0 L-- 10 - 2 rad.

Consequently, ^{and as shown in} figure ^{3, several}

consecutive reflections ( ~ 20) ^{cover a} length ^{of 10 cm}

after which (20 ^x 2 p 0) the laser beam returns back to the opposite direction and _goes out of the

« multipass system » approximately parallel ^{to the} incoming beam. This system permits ^{both to increase}

the light density illuminating ^{the atoms} pumped ^into

the Rydberg ^{state and} also, taking ^{into account the} divergence of the fast atomic beam (~ ^{5 x} 10- 3 rad)

to cover almost continuously ^the Doppler ^width.

Under this condition, the bandwidth of the resonance

induced by this second step of excitation is easily

estimated to be of the order of 10 GHz; this matches very well the spectral bandwidth of the multimode laser light. ^{All this} «multipass system » ^is placed

inside the vacuum chamber and can be precisely adjusted from the outside. Once a particular Rydberg

level has been selected the frequency of the multimode laser is, ^of course, kept constant at the optimal ^value.

In the geometry adopted ^{for the «} multipass system » the light shift induced by the broad band excitation is of opposite sign ^on each side of the true resonance;

therefore it will result in a small broadening of the line but not net light shift will _appear.

Fig. ^3.

Light« multipass system ^{». To} clarify ^the principle, ^the angles ^{and 0 are} expanded; ⁱⁿ practice fl r>5 ^{2.5 x 10-4 rad}

and 0 r>5 10-2 rad. After 10 reflections on each mirror the laser returns back in the opposite ^direction.

1170

A few cm further downstream the beam _passes

across the ionization region. ^{Two well} polished ^stainless

steel parallel plates forming ^{a 10 mm thick} capacitor,

as shown in figure 2, produce ^an electrostatic field of

typically ³⁰ kV/cm. ^{The ions} pass ^{across a} 3 cm dia- meter hole made on one plate and covered by ^{a flat}

stainless steel grid ^{of 60} % transmission. In order to minimize the background noise, ^{due to} the detection of undesired ions always present ^or accidentally produced inside the vacuum chamber, ^several impro-

vements have been made :

-

First a 5 mm metallic slit, ^{connected to the} ground, ^{has been} placed ^on the beam axis before the

capacitor plates. ^It permits ^to diaphragm ^{the atoms of}

the beam which _may hit the plates ^and produce

therefore parasitic ^ions.

-

As shown in figure ^{2 the} capacitor ^{has an addi-}

tional 3 mm thick step in its first part ^{where the} corresponding ^{electric field} falls down below the critical value [13]. Thus the neutral Rydberg ^atoms

pass first through ^a region ^{where the} electric field is sub-critical so that they ^{cannot be} ionized, ^{while the} residual ions of the beam are deflected and therefore eliminated (they ^{do not} reach the detector). ^The

increase of about 30 % of the electric field after the

edge ^{allows a} sufficiently high ^over critical value of the electric field to overcome the problems introduced by

the multiple ionization threshold phenomenon [14, 15]. ^{With this} geometry only ^ions arising ^{from the}

field ionization of the Rydberg ^{states are collected}

while those produced ^either by ^{collision or} by photo-

ionization before entering ^the capacitor ^are eliminated.

-

The ground ^connected capacitor plate ^{is cooled}

to liquid nitrogen temperature ^{in order to} improve ^the

vacuum in the detection region ^{and to} supress, this _way, undesirable ionizations due to collisions with the residual _gas. This last improvement enhances the

signal ^to background ratio : the part ^{of the} background proportional ^{to the} intensity of the beam is reduced by

two orders of magnitude ^{and becomes} less than 3 %

of the signal ^{while the} part ^{of the} background ^inde- pendant of the beam intensity ^{is reduced to less than}

2 counts/s.

3. Experimental conditions and results.

As mentioned in section II the first step of excitation involved in the experiment corresponds ^{to the tran-}

sition ’Sl/2, F = 2 -+ 2P3/2’ ^{F’ = 3} which is free from optical pumping effect If No is the number of neutral atoms per second coming ^{out the} charge exchange cell, ^{the mean number of atoms} populating

the 3p ^{level is} roughly given by ^the following ^{relation :}

-

where 5/8 corresponds ^{to the} statistical weight

of the F ⁼ 2 level of the ground state,

-

ACOD is the residual Doppler spectral width of the atomic beam (- ^{2 7r x 28} MHz) [6],

-

a is the saturating parameter; ^{a - 6.3 for a laser} intensity ^of typically ^{30 mW and a} laser beam diameter

of5mm,

-

and _{y is} the natural spectral width of the 2s - 3p

transition (y ~ ^{2 n x 10} MHz).

Under these conditions, ^{the ratio} N3p ^{N0 is found to}

be equal ^to 20 %.

The second step of the excitation is induced by ^a broad-band ^laser radiation. Its efficiency may therefore be roughly estimated from the Einstein coefficients [16], using ^the following ^numerical parameters ^which

correspond approximately ^{to our} experimental ^condi-

tions :

-

Energy spectral density p(v) N ^{8 x} 1 O-16 Jsm- 3 (for ^a typical ^{60 mW laser beam} intensity ^{and 0.25 cm2}

beam size).

-

Spontaneous ^emission probability from the 10 d

Rydberg level towards the 3p ^{level :} A (10d -+ 3p)~

4.6 ^x 105 S - 1.

-

Interaction time through ^{the «} multipass sys- tern» L ⁼ 1.2 _03BCs.

One gets ^an excitation efficiency 10d , N ^{N3p 2 x 10-3,}

where N 1 Od denotes the population ^{of the final} Rydberg ^level.

The choice of the _upper nd Rydberg level results in a

balance between the efficiency ^{of the} optical ^excitation

and the difficulty of the field ionization : the transition

probability which scales as n- 3 as well as the needed laser wavelength which falls in a more efficient region

for the available dyes favour lower n Rydberg ^levels;

however the critical electric field scales as n-4 and it

seems then technically ^{difficult to} go below n ⁼ 10 for which the corresponding ionizing electric field is

~ 30 kV/cm.

Thus, taking ^{into account} the transmission (~ ⁵⁰ %)

of the slit placed before the capacitor plates, ^the

transmission of the grid (~ ⁶⁰ %) ^{and the} depopulation

rate (~ ³⁵ %) ^{of the} Rydberg ^state (which depends

on the time of flight ^{of the} Rydberg ^{atoms from the}

laser interaction region ^to the detection region compared ^{to their radiative} lifetime) ^one gets ^{a theo-} retical overall efficiency ^{of ~‘ 4.1 x 10- 5.}

Figure ^{4 shows} typical results : the number of detected ions/s, ^{in the best} case, is equal ^{to 2.2 x 105}

for a measured atomic beam intensity ^{of 2 x} 1010 atoms/s. ^The corresponding efficiency ^{is therefore} equal ^{to 1.1 x 10- 5 which is} approximately ^{a factor 4}

lower than the above theoretical estimate. Probably

this is mainly ^{due to the} difficulty ^{one has to} adjust precisely the second laser beam in the ^« multipass systems. ^{The main} limitation of this method of detection comes therefore from the second step ^of excitation; ^{in order to} check this point ^more precisely

a second experiment ^{has been} performed ^{with the}

Fig. ^4a.

^{Detected ion} signal ^{as a function of the} post acceleration voltage applied ^{to the cell} (vi, v, and E ^are

fixed) ; v 1 is tuned to the hyperfine component F = 2 -+ F’ = 3 which is free from optical pumping ^{effect. Note the} presence of the satellite peak ^{on the} low energy side due to non-reso- nant neutralization process in the charge exchange ^cell [9, 10].

same experimental set-up ^{but with two} single ^mode

CW dye lasers both interacting collinearly ^{with the}

fast atomic beam. The first step of excitation is un-

changed ⁱⁿ comparison ^{with the} previous experiment

For the second step ^of excitation the internal cavity assembly ^is replaced inside the laser cavity ^{in order}

to achieve single ^mode operation. ^The frequency ^of

this laser is independently controlled with the same

sigmameter and the laser beam intensity ^is approxi- mately ^{1 mW} only. Under these conditions _up to 6 x 105 counts/s have been obtained with a neutral atomic beam of 5 ^x 108 atoms/s. ^This corresponds

to an efficiency ^{of 1.2 x} 10- 3, i.e. 100 times better than in the previous ^case.

5. Conclusion.

If we consider the particular ^{case of the} study ^{of the}

first ^resonance line, ^{detection of} fluorescence photons

has been proved ^to be very efficient and the proposed

method reaches only ^{about the same} sensitivity. ^But

for other resonance lines fluorescence detection is less sensitive and the proposed method could be

Figs. 4b, c, d.

Different plots ^{obtained at} various neutral beam intensities. The relative efficiencies calculated in each

case are gathered ^{in table I. It seems to diminish at low beam}

intensities. Such low neutral intensities were obtained by decreasing ^the temperature ^{of the source. Since the} experi-

ment was performed ^{without mass} selection, ^{at such low}

temperature ^{the ion beam did not} probably ^{remain a} pure

sodium beam, resulting ^{in an} apparent ^{decrease of the}

efficiency.

1172

Table I.

Efficiencies for ^{10d and 11 d} Rydberg levels measured at various beam intensities.

preferred. Comparison of the sensitivities of the two

proposed methods has shown a gain ⁱⁿ efficiency ⁱⁿ

favour of the second one; it mainly ^{comes from the}

more efficient use of ^the photons of the second laser

light : ^{all the} photons ^are simultaneously ^resonant

with the atomic beam which presents ^{a narrow} Doppler

width since the laser beam is now colinear. Moreover also the geometry is favorable : the laser light ^interacts

all along the beam with the atoms in the 3p ^state.

However this gain ⁱⁿ sensitivity ^is paid by the fact that

now both frequencies vi and v2 ^{must be} precisely adjusted ^and this method cannot easily ^be applied ^for high resolution spectroscopy ^on radioactive isotopes

for which neither v1 nor v2 ^are known. Moreover due to non linear effects and light shift, ^care should be taken in high resolution studies [8].

On the contrary this method is fully adapted ^{to the} study ^{of nd and ns} Rydberg series of francium [ 17],

the first laser (frequency Vt) being kept ^{at resonance}

with the known (7s ---> 7p 2P3/2) transition while the

frequency v2 is varied. Optical spectroscopy ^of Rydberg levels of francium is at the present ^time unknown and it would be interesting ^{to determine the}

ionization potential ^{of francium as well as the} quantum defects. Measurements on the nd series, ^{for which} important correlation corrections are predicted by ^the theory ^{are of} particular ^interest [ 18].

Owing ^{to its} high sensitivity the second method with two collinear lasers is _very attractive, but the first method (one ^{laser collinear} and the other perpendi- cular) may also be applied, specially ^{in the case of the}

first search of Rydberg levels, ^{if a wide} frequency

range has ^to be explored; ^{such a} frequency ^scan being

of course easier to perform ^{with a} multimode laser than with a single ^{mode one.}

The high efficiency ^{of the method,} especially ^with

the second geometry, encourages ^{us to} pursue with the field ionization detection technique. ^{Since the} problem ^{is now to} simultaneously adjust vi and _v2,

one can propose ^an alternative solution which consists

simply ⁱⁿ populating ^the Rydberg ^state directly ^from

the ground ^state by ^{mean of a} frequency ^doubled dye

laser. Such lasers indeed have been developed, ^these past ^few years; thanks to the use of intracavity ^non

linear optical crystals ⁱⁿ ring dye lasers it is possible

to get single ^{mode CW} tunable UV light. ^{As shown} by ^the present experiment ^{a UV laser power of a few}

milliwatts is sufficient. This does not exceed the

existing possibilities ^{of CW} frequency doubling.

Owing ^to the tools now available UV frequencies hyperfine spectroscopy ^{of the} ground ^{state is limited}

to the heavy alkali elements but if one is interested in the hyperfine spectroscopy ^of metastable states much

more elements can be studied, ⁱⁿ particular ^rare gases, and fortunately charge exchange ^{with alkali vapour is a}

very efficient _way to populate ^these metastable states

[19, 3]. Moreover, ^{in the} particular ^case of francium the proposed ^{method can be} applied ^to perform ^the optical spectroscopy ^{of the} np Rydberg ^series.

Preliminary experiments ^are in progress and appli-

cation to on-line produced ^unstable isotopes ^is planned

This experiment ^{has been} supported by ^{a DRET}

contract no 80-654.

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