Vacuum ultraviolet generation in phase matched carbon monoxide

Volume 43, number 4

OPTICS COMMUNICATIONS

15 October 1982

VACUUM ULTRAVIOLET GENERATION IN PHASE MATCHED CARBON MONOXIDE Fabrice VALLI~E and Jacques LUKASIK Laboratoire d'Optique Quantique du C.N.R.S., Ecole Polytechnique, 91128 Palaiseau cedex, France Received 29 July 1982

Resonantly enhanced four-wave sum frequency mixing in phase matched carbon monoxide leads to the generation of continuously tunable over 1200 cm-l coherent VUV radiation in the 1150 A range. Experiments allow to determine the VUV absorption cross sections and indices of refraction of CO. Multiphoton photolysis of CO has been observed and confirmed by the VUV absorption in atomic carbon.

Proper phase matching in frequency tripling or four-wave sum frequency mixing experiments can greatly enhance the vacuum ultraviolet (VUV) signal intensity and increase its tunability range. This has been largely demonstrated in metal vapors [ 1] or rare gases [2,3 ] but only scarce data exist for molecular systems [4]. Our Letter reports such 4-wave sum frequency mixing experiments in carbon monoxide where phase matching with xenon leads to the generation of continuously tunable over 1200 cm -1 coherent VUV radiation with peak powers estimated to be of the order of 20 W. Experimental determination of the VUV absorption cross section of CO as well as its index of refraction is also discussed. The energy level diagram for CO showing two- and three-photon resonant enhancement effects and the principle of our experimental arrangement were described in our previous paper [5]. Let us remind here that the two linearly polarized dye lasers 601 (5 mJ around 2900 A) and w 2 (30 mJ around 5600 A) are focused in a gas cell closed by a 10 mm thick LiF exit window. The 26o I frequency is set equal to the S transition bandhead energy of A 111 (v = 3) and 662 is probing in the neighborhood of the B 1 Z+ state producing light at 6o3 = 266 1 + 662. We previously described our low pressure results obtained in pure CO in a differential pumping scheme [5]. In a closed cell configuration the total pressure can be considerably increased and the generated VUV light presents new and interesting features which are 0 030-4018/82/0000-0000/$ 02.75 © 1982 North-Holland

shown in fig. 1. Table 1 indicates optimized pressure ratios of CO : Xe experimentally determined at the peak of each partial tuning curve ( A - K ) in fig. 1. The total tuning range of w 3 extends now about 1200 cm -1 between 1142 and 1159 A and is, at present, limited only by the tunability of the w 2 dye laser (Rh 560). The registered total energy spectra exhibit, in general, increasing blue shifts with increasing pressure of pure CO. The 603 signal, however, sharply drops in the region of 87080 cm -1 which corresponds to the last clearly identified R(33) transition of the B-X (0-0) band [6]. On the red side of the P branch no 663 signal could be generated at any pure CO density. The VUV was again efficiently produced on both sides of the B-X band by phase matching with xenon which is negatively dispersif between 1135 and 1170 A [7]. On the contrary, adding xenon in the central region (curves G-H) decreased the VUV signal which confirms that CO must be negatively dispersif there. The addition of positively dispersif argon provokes only slight enhancement of the VUV intensity. Also further increase in the CO pressure leads to strong reabsorption of the 663 signal. Such reabsorption permits, in fact, to evaluate o(w3), the absorption cross section at 663 of CO. Bjorklund showed [8] that two focused beams produce in a negatively dispersif medium sum frequency radiation with the intensity Iw3=2~1+o02

oc 2 2 (3) 2 Ioa, I w 2 N IX I F(z2tk),

(1) 287

Volume 43, number 4

OPTICS COMMUNICATIONS

15 October 1982

30

•~

!

20

_=

i 0116300

81)600

i

m

2w~ .t~ 2

$750Q

872OO Total

energy

(cm-l)

Fig. 1. Tuning ranges and relative intensity (not normalized for Ico2) of VUV radiation in different mixtures of CO : Xe (see table 1). The shaded curves F, G and H represent symbolically enhancements due to P and R branches of the B-X transition with G(b) and G(c) corresponding to their respective bandheads. (See, for more details, ref. [5 ] ). Atomic carbon absorption is designed by C(a). Table 1 Experimental parameters for phase matched c~3 generation in carbon monoxide (see fig. 1) Pression (torr)

CO Xe

Curve A

B

C

D

E

F

G

H

I

J

K

150 40

100 35

82 35

50 33

45 43

5 10

4 0

80 0

80 23

80 45

80 65

where N is the number density of the gas used, ×(3) its third order nonlinear susceptibility and F is a factor depending on the phase mismatch zXk = k 3 - 2k 1 - k 2 with k l , k 2 and k 3 being the wave vectors (k = 27m/X) at frequencies COl, 6o2 and 603 respectively. Taking into account reabsorption effects eq. (1) should be replaced by

,l 2Ngol,g?)L 2

X F(ZXk)exp[-oCO(6o3)NcoL],

(2)

with L being the distance between the focus and the exit window of the cell. Eq. (2) is valid only if one can neglect: 1) xenon absorption at 6o3, 2) third order nonlinear susceptibility of xenon in comparison to carbon monoxide. Evaluating OCO at, for example, 6o3 = 87170 cm - 1 288

we note that the closest absorbing level of xenon is more than 1000 cm -1 away and calculating the susceptibilities we find X~) ~ 2 × 10 -34 esu >> X~) 10 -36 esu, which is confirmed in our experiments through observation of Iwj (CO) > Iw3(Xe) and justifies eq. (2). Using this equation we can experimentally determine the absorption cross section of CO at 6o3 by measuring I w at different pressures of CO in CO: Xe mixtures a n ~ p l o t t i n g log(Ito ~ IN20) as a function of N c o . At 87170 cm - 1 (~t~c = 1147.2 A) we find OCO ~ 6 × 10 -20 cm 2 in good agreement with the value obtained in synchrotron photoabsorption measu~rements by Lee and Guest [9] who estimate Oco < 8 X 10 -20 cm 2 in this spectral region. From the same experiment we can also derive the k vector mismatch per atom CCO , between the generated radiation at 603 and the driving polarizations

Volume 43, number 4

OPTICS COMMUNICATIONS

15 October 1982

which is defined by [7] Ak = C c o N c o + CxeNxe.

(3)

The optimisation of the phase matching factor F imposing blkk = - 2 [8] we have

2~ bCco

PCO-

A

CXe CcoPXe,

(4) 10

where b is the confocal parameter and ~ = piN ,,~ 2.8 × 10 -17 torr at 20°C. Furthermore, from the slope of the fig. 2 we obtain Cxe/Cco = - 4 . 6 6 at X~ae = 1 147.2 A. The value of CXe was calculated at this wavelength for the case of third harmonic generation [7]. Knowing the indices of refraction for xenon at 601 and 60~ [10] Cxe is found in our experiment to b e - l . 5 3 ,~ 10 -17 cm 2 and CCO = +3.3 X 10 -18 cm 2 at the above wavelength. This last value easily allows the determination of the index of refraction of CO at 603 . Using the experimentally obtained value o f n 1 at 601 [11 ] and n 2 derived from the Sellmeier

tin

86519 crn"1 2 p2 Zpo _ 5d 3P10 [1155.8

A]

2~11 +(~2 0

L 86500

i 86520

Total energy /cm-1/ Fig. 3. Atomic carbon absorption profile at 86519 cm-1. PCO = 80 tort. 250

equation at 602 [10] we find for CO (n 3 - 1) = 5.2 × 10 - 4 at 1 147.2 A. Fig. 1 (Ca) and 3 show an interesting feature of absorption of the VUV signal at 86519 cm -1 . Atomic carbon energy levels [12] identify this frequency as corresponding to the 2p z 3 P j = 0 - 5 d 3P~j=1 transition. The atomic carbon is most probably produced in our experiments through a multiphoton photolysis of CO involving three 601 photons of the total energy of 12.86 eV:

200

Jf,

180

0 U

'too

O.

CO{v = 0, x l ] ~ +) + 3h60 1 ~ C~3Pj=0 ) + O(3p).

/ Xe Pressure (Torr) Fig. 2. CO pressure as a function of Xe pressure for different phase matched CO : Xe mixtures at h~ac = 1147.2 A.

(5)

A similar process was observed by Bokor et al. [13] using two photons and an ArF* laser. The region of 12.8 eV is rich in numerous valence and Rydberg states [14] which can significantly contribute to the photodissociation of the CO molecule. Such photodissociation is facilitated in our experiments through the two-photon resonant enhancement o f the o = 3 level in the A 1 I1 state. The slow buildup of photolytic products (black powder due to C 2 molecules) noticeable in our cell is another confirmation of the process. 289

Volume 43, number 4

OPTICS COMMUNICATIONS

15 October 1982

10

I 5 I ~C

i i

[

I

'It0

I

I lO I E

I i

I I

]

I i

I

I

t 10

I I Illltl Q 1 i i i ii P

°~

3" 4.

N

2W2-W! ?o~oo

?o4so Total

?o.~oo energy

7o~so

(cm4)

Fig. 4. Generation of co'3 -- to x + 2co2 showing enhancements due to P, Q and R branches of the A-X (4-0) transition, to t is fixed and co2 is scanned at PCO = 3 torr. It is interesting to note that no other absorptions originating from other than J = 0 levels in the 2p 2 3p state were observed. This seems to indicate that our photolysis process in CO selectively creates the atomic carbon in its fundamental state with J = 0 only. The efficiency of atomic carbon production can be estimated from the absorption linewidth using the curve of growth method [ 15]. In an experiment carried out at PCO = 80 torr, we estimate the density of atomic carbon to be of the order o f 1013 cm -~3 which corresponds to the efficiency of 3 X 10 - 6 . Our experiments also produce coherent tunable emissions in a few regions around 1400 N when the sum frequency co~ = ~o1 + 2w 2 is tuned in the vicinity of v = 4 and v = 5 levels of the A I I1 state. Fig. 4 shows the generated signal around 1420 A at p = 3 t o r e Increasing the CO pressure in this region leads to a considerable extension o f the tunability range which exceeds 350 cm - 1 . Its blue shift at higher pressures, consistent with the vector phase matching requirements, confirms similar recent observations o f Glownia and Sander [ 16]. Moreover, resonant enhancement of the generated signal due to P, Q and 290

R branches o f the A-X transition is clearly seen and demonstrates again the useful spectroscopic application c f the sum frequency mixing experiments in the VUV. It is worth noting in conclusion that phase matched carbon monoxide can become an extremely useful source o f continuously tunable coherent VUV radiation between 1400 and 1100 A and even beyond in the XUV region. The rich spectrum of rovibronic levels in a dense electronic manifold (singlet states such as A 1 If, B 1 E, C 1 •+ or E 111 and numerous triplet states) combined with the simple frequency mixing technique can produce a very high brightness radiation useful and competitive for many different applications.

References

[1] J.F. Young, G.C. Bjorklund, A.H. Kung, R.B. Miles and S.E. Harris, Phys. Rev. Lett. 27 (1971) 1551. [2] A.H. Kung, J.F. Young and S.E. Harris, Appl. Phys. Lett. 22 (1973) 301. [3] R. Hilbig and R. WaUenstein, IEEE J. Quant. Electron. QE-17 (1981) 1566.

Volume 43, number 4

OPTICS COMMUNICATIONS

[4] K.K. Innes, B.P. Stoicheff and S.C. Wallace, Appl. Phys. Lett. 29 (1976) 715. [5] F. VaU6e, S.C. Wallaceand J. Lukasik, Optics Comm. 42 (1982) 148. [6] S.G. Tilford and J.T. Vanderslice, J. Mol. Spectr. 26 (1968) 419. [7] R. Mahon, T.J. Mc Ilrath, V.P. Myerscough and D.W. Koopman, IEEE J. Quant. Electron. QE-15 (1979) 444. [8] G. Bjorklund, IEEE J. Quant. Electron. QE-11 (1981) 287. [91 L.C. Lee and J.A. Guest, J. Phys. B: At. Mol. Phys. 14 (1981) 3415. [10] InternationalCritical Tables, Vol. VII, ed. E.W. Washburn (McGraw Hill, New York, 1930).

15 October 1982

[ 11] P.L. Smith, M.C.E. Huber and W.H. Parkinson, Phys. Rev. A. 13 (1976) 1422. [12] C.E. Moore, Selected Tables of Atomic Spectra (CI-CVI), NSRDS-NBS3, Section 3, US Dept. of Comm., November 1970. [13] J. Bokor, J. Zavelovich and V.K. Rodes, J. Chem. Phys. 72 (1980) 965. [14] S.V. O'Neil and H.F. Schaefer III, J. Chem. Phys. 53 (1970) 3994. [15] A.P. Thorne, Spectrophysics (Chapman and Hall Ltd., London, 1974). [16] J.H. Glownia and R.K. Sander, Appl. Phys. Lett. 40 (1982) 648.

291