Laser induced radiative preassociation and isotope separation

Volume 37, number

CHEMICAL QHYSKS LETTERS

3

-LASER INDUCED

RADIATIVE

PREASSOCIATION

1 February

1976

AND ISOTOPE SEPARATION

Conrad SCHMIDT Brown Boveri & tie, Zeutraies Forschungrlabor, Heidelberg, FRG Received 8 September

A separation

1975

of isotopes

produced

by selective de-excitation

Consideration of chemical lasers [ 1,2], laser induced chemical reactions [3], and induced radiative preassociation [4] led to the present proposal for isotope separation. Many photochsmical and photophysical methods [5,6] resulting in isotope enrichment by the use of intense CO, and dye lasers have been suggested. The known methods act at the input channel of a chemical or physical reaction so that their action is noted at the left-hand side of the reaction equation. We will consider how to influence the output channel of a chemicai reaction and will be concerned with the shortening of the radiative lifetime of a chemically pumped species by means of an inducing radiation field. Usually, the final or intermediate products of a chemical reaction are electronically and/or internally excited. The rate at which a stable molecule is rormed is predominantly determined by the inverse process of fomlation, since only a few stabilizing processes such as radiative transitions or er.erB’ transfer to a third colliding particle are possible within the collision time of 10-12-20-13 s. The radiative lifetime, however, may be decreased by stimulated emission

(X = wavelength, / = effective intensity, T = radiative lifetime) provided that the lower state involved in stimulated emission can be removed rapidly enough. This supposition is usually fulfilled by fast relaxation and atom exchange reactions. An important aspect of laser induced preassocistion is the waveiength or isotope dependence of the lifetime shortening. ‘The lifetime I depends reciprocally on the light intensity and its shortening by many 574

of chemically

pumped

species is outlined.

orders of magnitude can be effected by means of powerful lasers. Consequently the radiation rate constant k, = l/r of the general reaction mechanism kr AB+Cc{BC}+A,

0)

{BC) +-BC+hv, kr

(3)

x-q {BC)+M--+BC+M,

(4)

may compete easily with the fast dissociation kd of the highly excited collision pair { BC) resulting in the enhanced formation of stable BC molecules. In the same way the induced radiation, k,, may determine the de-excitation of the collision pair rather than the three-body quenching process, k,. We now formulate the ability for isotope separation by laser induced radiative preassociation: the lifetime of the highly excited collision pair involving the wanted isotope is selectively shortened to foml a stable molecule while collision pairs with the undesired isotopes succumb the undisturbed reaction mechanism. So most collision pairs redissociate into the reactants. Since the undisturbed mechanism produces stabilized molecules as well it is a question how strong the enhancement of the desired reaction need be to perform a perfect separation. In favorable cases the selective decrease of the natural lifetime achieves isotope separation in a single process. If not, we regard the laser induced preassociation as the initial step of an extensive mechanism containing different reaction paths for the excited and de-excited collision pairs as well as for the reactants.

1 Fcbruruy 1976

CHEMICALPHYSICSLETTERS

Volunie 37, nuniber 3

Reaction

Coordinate

-

I

Reactants

F&. 2. Reaction coordinates fluoride in various vibrational intcmucledr

distance

Products -

.&&a~$

the formation states.

for

of hydrogen

*

Fig. 1. Radiative preassociation.

Fig. 1 describes the events of the radiative preassociation in the case of two atoms. During association the electronic states A, B, and C are populated as well as the ground state with different distributions over the vibrational levels. The B state does not correlate with ground state atoms. however two paths allow for B state population via the A state by internal conversion in 3 three-body process or via the C state through inverse predissociation in a two-body coliision. Now radiative transitions occur between the B and A states or directly from the C state. For allowed transitions the lifetime is 1O-6-1O-8 S, being considerably longer for metastable species. With eq. (1) the changed lifetime is between lCYIO and lo-*” S. Therefore, the radiative rate constant approaches the rate constant of the inverse formation process of the order lOl3 s-l. As another example fig. 2 describes the Formation of hydrogen fhroride

formation process k; which is almost equally large as kv. Selective stabilization OFthe products having an internal energy of AH + E, (cf. arrows in fig. 2) could favor the distinct product formation. For both examples it is necessary to remove the de-excited species quickly enough from the scene before chemical or physical quenching and relaxation interfere. The basis of the present proposal is the isotopic shift of the atomic or molecular states. The electronic states of the atoms are shifted by about one wavenumber. For molecules addition~ly there is the shift of rotational and vibrational levels being several times one wavenumber. Thus optimum isotope separation requires a light source with a narrow bandwidth corresponding to the emission bandwidth but essentially smaller than the isotopic shift of the involved molecular species. All photophysical or photochemical methods known so far are based on ‘he selective excitation or ionization in a singie or multiple photon process. A!though excitation and ionization are isotope specific, offering a 100% separation possibility, the efficiency of the real separation is reduced by energy transfer or charge exchange among the species of the isotopic mixture. For instance, the rate constant for ground state molecules

in several vibrational levels. Relaxation processes cause distinct rotational-and vibrational distributions which allow for the construction of a’chernical laser due to the given inversion. The rate of formation of HF is determined by the rate constant of the inverse

k, = A exp (-E*,/kT)

,

(5)

with E, = activation energy; regarding the vibrationa! excitation: k; =A exp [-(EA -i~~,ib)fkTf

,,

(6) 575

Volum

37, numbzr

CHEMCAL

3

PHYSICS LETTERS

where huVi, =. vibrational energy. Therefore the enhancemenibf the chemical reaction owing to selective heating by infrared lasers is characterized by the ratio _

k;/kf% exp (11Yyib!kT) . Becsuse kil/kil

of relaxation

01

the ratio of rate constants

=Z exp I~(~A/X-T)

(vi2 -Yil)f(Q

+ Yil))

is [7] I

(8)

where il and i2 indicate the two competitive isotopes. For typiCal data (EA = 3, ev; /lvvib = 0.1 ev (Co-laser), T= 300 K) this ratio yields kil/ki2 z 10. ~gu~ents about relaxation as in relation (8) are not thrown into relief for the present problem. For an explanation we compare the situation of eq. (8) with the present one. In the firs: case vibrational energy exchange between the isotope modi~cation of the molecules is restricted to the lowest levels 0 and 1. The rapid quasi-resonant energy exchange reduces the selective initiation of the chemical re,action, whereby both the selectively excited species am the isotopic modifications tixcited by vibrational energy exchange pass over the activation barrier with about equal efficiency. In the present case vibrationa! ~u~nturn transfer between the isotopic modifications of the molecules cannot injure the separation efficiency as eq. (8) demands. Then the distinctiv: mark of the present method consists in the seiective fo~ation of mofecules soMy by induced radiation. There is no efficient reaction, particularly no energy transfer reaction, which poptilates the high vibrational levels of the unwanted modi~ca&io~ of the collision pair at the dissociation threshold at aLl.Relaxation plays its role in that the unwanted modification is formed by the nonefticient natural pre2ssociation connected with stabilization owing to vibrationa energy exchange. Thus relaxation wii: influence the separation efficiency if the selectively formed modification cannot be “precipitated” fast enough.

1 Februruy

1976

A first step towards realizing the above proposal has been made. For a particula; system, i.e:, the chemi!uminescent recombination of nitrogen atoms it has been shown [4] that laser induced prea~ociation is possible. T%e measurement was carried out as an absorption/amplification experiment by shining laser light through active nitrogen. For the transition N1 was (B 3I-& v’ = 12 + A31$, u” = 8) ~pli~cation

observed as well as for the u’ = I1 + LJ”= 7 transition while lower levels showed ahsorption. The population of the U’ = i2 level at the dissociation threshold occurs via inverse prediss~ciation whereas population of the v’ = 11 level involves relaxation from u’ = 12. The lifetime shortening was about 4 orders of magnitude. In the r;ase of nitrogen recombination, the chemiluminescence lies in the visible, a favorable spectral region with regard to tunable dye lasers. The distance of the 12 + 8 from the 11 -+ 7 emission line is about 50 A. The isotopic shift between 14N2 and 14NL5N or 15N, amounts to 6.1 w or 11.8 A respectively which is indeed larger than the bandwidth of the dye laser. However, nitrogen recombination is not a favorable system to demonstrate isotope separation. More suitable molecules are NO, CO, CN or tri-atomic molecules which are easier to “‘precipitate” after induced formation.

References [ I] K-L. Kompa, Topics in current chemistry, Vol. 37 (Springer, Berlin, 1973). (21 A.N. Chester, Proc. IEEE 61 (1973) 414. [3] J.T. Knudtson and E.hI. Eyring, Ann. Rev. Phys. Chem. 2.5 (1974) 255. (41 C. Schmidt and 51. hicister, to be published. fS] F.P. Sch%fcr, Plennrvortrag, Fr~hj3hrs~~~~n~ DPG, Kijin {1975), invited paper. [6] V.S. Lctokhov, Science.180 (1973) 451. [7] EM. Belenov, E.P. Markin, A.N. Orac