Laser spectroscopy ofD3

23 July 1982

CHE~ilCALPHYSICSLETTERS

LASER SPECTROSCOPY OF D, i-I. FICGER, H. MOLLER, W. SCHREPP hfax-P~rlck-lrlstltlitfiir @uatitenoptll;, 8046

Federal Reptrbhc of Germat~y

~dFThlllg,

and H WALTHER

Rcce~vcd1 May 1982, in final form 1 June 1982

The D3 wos rormed tryP ~IOIIOW cathode dnchnrgc m Da Transitions between the slates 3p2E’, 3p7Ai and 2s ‘Ai wre stlmulatcd by dye-lnscr rodiatlon

The rcson~nccs were momtored by obsemng

the larcr-mduccd

change of the cnuss~on

rrom selected lcvcls of Da_ Many nebvImes WCICobserved and assyncd

I. Introduction The molecules H3 and I?3 were first assumed to occur ,IS actlvdted complexes in the fundamcnral reac-

tions between Hz and H and D, and D. Several papers have been publisl~ed on computations of’ the potential surface of the electronic ground state of H,, beginning with the early work of Eyrmg and Polanyi [I] and ~ontlnuing recently w&h the accurate ca~culatlons of Slcgbahn and Liu [2] _ The first theoretical study of the potential surfaces of excited electronic states of H, was carried out by Frenkcl [3] He perrorrned restmted Hartree-Fock calculations to mtcrpret collrsion experiments of Hz with metastablc H(2s) atoms [4]. Deep rnimma III the energy surfaces for Djh symmetry of the complex H3 were found , The tuneof-flight expcrmznt of Vogler [S] *on a predraocutting H3 complex formed in collusions of H; with Hz gve the first experinlent~l evidence of quaslbound states m H, However, the existence of the simplest polydtomlc molecule was finally established by Herzberg et al. [7-101 when Ihc emission spec* For

CorrcCt~olLs

161. 90

Of th

L”tCIQ~Ct~tlO~

U-I thiS paper.

SCC Id.

tra of H3 and Dj were observed m a hollow cathode discharge in H, and Dz, respectively First Henberg found bands In the wavelength regions around 5600 and 7100 &The hnewidth of the transitions was ~10 A, being much larger than that of the H, lines, which cover the whole spectrum from the IR to the W These and other sim;lar bands could be intcrpreted as being emltted by one electron moving in

the field of a G core with D,, sy~lmetr~. The bands at 7 100 and 5600 14were found to hzve the Rydberg states 3p 2E’ and 3p2Ai as upper states and are rotationally spht m adhtion. The equal widths of the Imes of the two bands indicates a common predissoclating lower level of the transitlons, asslgned as 2s ‘Ai- The predissociatlon is caused by the mtcraction of this level with the ground state in which the molecules dissociate mto HZ(‘X;) + H(n = 1) Herzberg’s work stumtlated theoreticral studies on the H, and D3 system. Hartree-Fock cal~u~ations using the recent rest& on the @ core of Dykstra and Swope [ 1 l] and Carney and Porter [12] were performed by drfferent authors [13-S]. In partrcuiar, the energies of the Rydberg states with IZ= 4 [13,14] and n =S [14], not observed expedmen-

tally so far, were given. The knowledge of their 0 009~2614~82~0000-~~~~

02 75 0 1982 No~.Ho~~d

Volume 90, number 2

energies as well as their litetlmes would be most valuable for a deeper understanding of this molecule A laser spectroscopic method allowmg high resolution was therefore developed. So far we have investrgated the Dg bands at 5600 and 7100 A

2 Expenmental

23 July 1982

CHEMICALPtIYSICS LETTERS

arrangement

The hollow athode as used by Hetzberg and coworkers was modified for the laser spectroscopy experiment (fig 1). By imaging the chfferent parts of the plasma of the hollow cathode on the entrance sht

essary because of the small overlap of the spcctrdl chstnbutron of the laser with the profile of the lint to be induced The laser was tuned with a brrefringent fdter and had a spectral width of =O 3 A. A krypton 101-1 laser (7 100 8, band) and an argon ion laser (5600 A band) were used to pump the dye laser. A chopper

Inside the canty modulated the laser I& for phasesensitive detection of the laser-induced signal The hollow cathode is water-cooled. FormatIon of D, molecules is maximal when the D, gas pressure in the tube IS 1.1 Torr and IS rather mdependcnt of the gas flux. However, It has to be hlghcr than IO seem. The discharge current IS ~120 mA.

of a monochromator it was found that most of the Dg light was emitted close to the axis of the cathode A hole of 2 mm waz therefore bored in the centre of the closed end of the cathode so that the plasma could be lrradlated by a laser beam The tube was put inside the laser resonator to have higher intensity The Interaction region between the laser beam and dlscharge plasma could be observed through a sht in the dlrection perpendicular to the axis of the cathode 1 was focused on a 60 cm monochroma(fig 1) n le s1.t tar whose output was monitored by a photomultiplier Rhodammc 700 and 110 dyes were used for the

mvcstigatlons on the 7100 A band and the 5600 A band, respectively. The power inside the laser cavity, and thus in the cathode plasma, was estimated to be 20 and 10 W, respectively. This high power was nec-

3. Expenments

and results

The usual method of laser-induced fluorescence to study term energies and hfctimes of unknown molecular levels cannot bc. apphed in the case of HJ and DJ because of the bright background hght cluscd mamly by the radiation of H2 or D2_ A method more selective for the Dj lines therefore had to bc used. The monochromator selected a narrow part of the D, cmlssion spectrum contamlhg one or a few hnes and the Izcr was tuned. If the laser wavelength coincided with a transition starting from the upper level of a Dg lmc observed wrth the monochromator the photomultiplier signal changed since a population altemtron was mduced by the laser radlatlon. L&t resultmg from

Anode

Fig. 1 Hollow othode usedm the hscr expcrimcnts 91

CHEMICAL PHYSICS LETTERS

Volume 90, number 2

23 July 1982

FIN. 2. Em&on spectrum of D,. The perpendicular band around 7100 A is shown. The broad lines belong to Dj, the narrow ones to Dz. For a few peaks the asvgnrncnls of the D; Lirles is Bven The urow designates the transitions observed using the 60 cm

monochromotor, when the laser-induced spectmm of fig. 3 was taken.

D, as well as other D3 hnes is not influenced by the laser rachation II the fluorescence from their upper levels is not observed by the monochromator. The influence of the background radiation IS therefore suppressed The experiments described m this paper were performed on D3 instead of H, because of the clearer and more intense structure of Its spectra Comparison of the spectra shown in figs 2 and 3 clearly demonstrates the advantage of our method. Fig. 2 shows the emlsslon spectrum of D, measured by a 60 cm monochromator with a resolution aX/X = 2 X 10B4 in the wavelength range 7000-7400 A. According to Herzberg and Watson [9], it has to be Interpreted as the

r

spontaneous decay of the many rotational sublevels of the Rydberg 3p2E’ electronic level. Due to the predissociation of the lower level, the D, lines have a width of 5 A. In the previous papers this property has been used to distmgulsh the Dg lines from the many D, lines. In fig. 3 a laser-Induced spectrum IS shown for about the same wavelength range. For thus measurement the monochromator was tuned to the lines rR25, ‘R13, and ‘R02 and therefore only transitions starting from the upper levels of these transitions, namelyK=3,N=6;K=2,N=4;andK= l,N=3, respectively (A’,K quantum numbers of a symmetnc top) show up in the laser-induced spectrum. The measurement displays only five lines as expected according to the selection rules. The noise between the hnes ISmainly due to the discharge noise. The spectral lines observed correspond to a decrease of the observed intensity, i e. a reduction of the population in the upper levels by stimulated transitIons_ This means there is population inversion as expected owmt to the predissociation of the lower levels With the method used the complex spectrum of fig 2 can be decomposed into many simpler ones with completely resolved lines, and sorted according to the upper levels, being preselected by the monochromator. Table 1 shows the results for some monochromator settings. The laser-induced lines could easily be assigned by comparing their wave numbers with those calculated from the refmed symmetric-top model for D, as gven in ref. [9]. The mean deviation of the calculated wave numbers of the transitions from the experimental ones is 4 cm-‘. As the experimental

T!fJJ!h ‘P,5

‘P27

‘cl,L

‘cl26

7150

7200

7250

7300

Laser Wavelcnglh

FI&

3.

Pxt of the laser-induced

7100 Aband. The monochromalor ‘R25, rR13,‘Ro2

92

7350

7

[AI -c

spectrum oiD3 in the momtored Ihe tunsltions

Volume 90, number 2

23 July 1982

CHEMICALPHYSICS LETTERS

Table 1 Laser-induced transItionsm the 7100 A band ofD3. The fust row gives the monochromator rctting, the second row the emu~on Uansinons observed for this scttlng. Tnnntions observed for the frst trne are designated by a stm m the last row Emission observed

“em (cm-‘) 14187

14159

Laser-mduccd transitlons translhon

“&

‘Ro4

13687 9

‘R,5

13600.1 13904 0 13518 2 13836.6 13977 8

‘Ro6 ‘Ro2 ‘R,3

(cm-’

13752 9 ‘R25

14150 14128 14120

13968

)

assgnmcnt

ucalc (cm-’ 1

rPo6 ‘P,7

13692.6 13602.0 *

‘416

139125 *

‘Pa8 ‘PO4

13524 2 * 13839.6

‘C!14

13976 9 *

‘PI5

13750.4

13890.4

IQ26

13890.6 *

13579 1

‘P27

13579.5

‘R76

13498 4

‘PZB

13501.0 *

‘R27

134115

IP29

13416.5

l

‘R24

13918.8

rQz5

13921.6

l

13649.2

‘Pz6

13651 B

13942 4

‘424

13945 4

13713 0

TPl5

13718.4

13888 1

‘Qs5

13091.3 13620 5

IR23 ‘R,4

13618.0

‘P,6

PQ23

13936 9

PP24

13939.8

‘Rq6

13802 2

‘Qq7

13804.5 +

‘R47

13367 8

‘P&J9

13374 2 *

PP45

14197 8

pQ,4

14197.3 * 141019

rP,Z

14098 1

rRoO

rR23

14098.1

rR22

14100 9

13945

‘424

14123 3

‘R23

14128 3

13715.8

‘pz5

13718.4

13923

PP,5

14150.7

pQ,4

14151.3

13864

13839

13620

l

pP68

14223.2

pQ,7

14223.4

TQJ6

14133 3

TR,S

14134 7

pP,6

14133.3

pQ,5

14133.6

rQ45

14081.5

‘R44

14089 2 *

PP,9

14238 7

PQ,8

14242 7 +

‘PO4

14158 0

‘Ro2

14159.7

rQ46

14104 1

‘R45

14107.6

‘p36

14117.4

‘R,4

14119.0

uncertainty is of the same magmtude, no least-squares fit was performed with the new data to improve the molecular constants Many lines @en in table 1 have not been observed in the previous papen by Herzberg and co-workers [9] _

In the same way the D3 band at5600 A was investlgated. In the cmlssion spectrum of lhe hollow cathode the P and R bnnches an be resolved; however, all the lines of the Q branch overlap as shown m ref [S]. Wdh our method WCare able to resolve 93

Volurnc 90, number 2

CHEMICAL PHYSICS LETTERS

23 July 1982

transitions to the n = 4 and 5 levels are induced by the laser radiation. These experiments are now under. way The tranitlons from n = 3 to II = 4 and 5 correspond to wavelengths between I and 2.5 pm as wlculated by King and Morokuma [I 3f These Infrared transitions should also be detectable by observing the hght of the II = 3 to n = 2 transitIons, which are in the visible spectral region

Acknowledgement

1

We would like to thank G Herzberg for many helpful discussions and numerous useful hints concerning the construction of the hollow cathode The technical assistance of H. Kasbauer and K. Fntsch is hatefully acknowledged.

,

5550 Laser

5600

Hovelenglhri1

rig. 4. Part of the Inscr-mduced spectrum olD1 m the 5600 A band Here the monochromator monitored the P(4) tra.nsitlon

completely &IS part of the spectrum A singIe Q hne ISshown in Iig 4 Energies for smgle Q lines as determined III our expenment are given in table 2 They agree with the calculated values g,wen III ref [8] withm the experimental error of 4 cm-‘. The method descrtbed should also be apphcabie tf Table 2 Laser-Induced Q transitions of the 6500 A band ofD3. Emw ~lon from the corrcspondmg P tra-atlons hzs been momtored. The calculnled values are taken from ref_ [8].Thc cxpcrlmental un~rl~~nty 1524 cm-’

References [I j H. Eynng and hi Polanyi, 2 Physikc.Chem (terpzg) El2 (1931) 279. [2) P. Siegbahn and B Liu, J. Chcm. Phys. 68 (1978) 2457. [Ii] E. FrenJcel, 2. Naturforsch. 25a (1970) 1265. [4] FJ. Comes and U.Wcnnmg, 2. Naturlonch. 24a (1969) 1227 IS] M. Voglcr, Phys Rev. A19 (1979) 1. [fi] J.K.G. Watson. Phys. Rev. A22 (1980) 2279. [? J G. Henberg, J. Chem Phys 70 (1979) 4806 [8] I Dabrowskr and G Herzber.g, Can. J. Phys. 58 (1980) 1238. [9] G. Herrberg and JX G. Watson, Can. J. Phys. 58 (1980) 1250 [lo] G. Herzbcrg, H Lew, JJ.Sloan and J K.G Watson, Can. 3 Phys. 59 (1981) 428 [I l] C.E. Dykstra snd W.C Swopc, J Chcm. Phys. 70

Transition

“ohs (cm-‘)

uwlc (cm-’ )

(1979) 1 1121G D.Carncy and RN Porter, J Chcm. Phys 65 (1976)

Q(1) Q(2) Q(3)

17868 3 178625 17859.1 17849 2 17842 8

17869.4 17864.7

3547. [13] H.F. King and K. hlorokuma, J.Chem. Phys. 71 (1979)

17860.1

3213. 1141 hi. Jungen, J. Chem Phys 71(1979) 3540.

Q(4) Q(s)

94

17854.2 17884 9

Il.51 R.L. ~~~~n, J.Chem Phys. 71 (1979) 3541.