Ion dip spectra toluene and aniline in a supersonic free jet

Volume91, numbcr 5

CHEMEAt PHYSICSLCTTERS

24 September 1982

ION DIP SPECTRA OF TOLUENE AND ANILINE M A SUPERSONIC FREE JET

Jun-ichi ~lU~~~1, Kojl KAYA * and Mitsuo IT0 DeportmentofChemistry, Famlty of Sclerrce. Td~ohu Unwenity. Ser&r 980,Jupon Rccclved3 May I982

The iOn dip spcctn of mlucnc nnd onlhc wcrc mcrsurcd usmg n supcrsan~c Frco jet. It W;)S JC~O~SW~WI that 0” aon dip spectrumgrvcsa rauo of the tnnsit~on probability from a ground state to a rcsonan!mtermcJ&e state 2nd that from the

mtcrmcdtatestate to the mmzattoncontmuum.

1. Introduction High sensitlvrty of detection of multiphoton ~omzation (MPI) spectroscopy has been proved quite effective in the study of excited electronic states of molecules [ 11. Owing to the advantage, the MPI technique contains the possi~llty to develop into novel spectroscopjc methods and one of them is ion dip spectroscopy wtich was recently reported by Cooper et al. [2]. In ion dip spectroscopy one observes reduction of ion current. The ion current is produced by pumping an electronic excited state and the sub~quent MPI induced by 3 pump laser. The ion current reduction is due to depopulation of the pumped level by stunulated emtssion induced by a second probe laser [2]. When the stimulated transition occurs between the resonant intermediate state and the ground state, one can obtain an 1011dip spectrum that corresponds to the SVL fluorescence spectrum from the intermediate state. In this communication we report the ion drp spectra of two large polyatomic molecules, toluene and amhne, obtained by using a supersonic free jet. Their first ehcited smglet states (Sl) were pumped and the stimulated transitions to the vibrational levels in the ground state (SO) were observed as ion dips. Advantages of the use of a supersonic free jet in ion dip spectroscopy were discussed. By compare the ion dip spectra with the SW., * Presentaddress:Departmentof Chematry, Faculty of Scicncc and Engmeetmg,K&I University, Hlyosht-machr,Kohoku-ku, Yokohama, Japan

0 009-26 14/82~0000-0000/!3 02.75 0 1982 North-Holland

fluorescence spectra, the ratlo of the transition probabdities of So-S, and SI -lonizatlon continuum was obtamed.

2. Experimental In fig. I is shown the schematic diagram of the cxperimental apparatus. 20% of 3 nitrogen hscr (Molcctron W 22) output was used to pump a home-made dye laser (fwhm % 3 cm”). It was then introduced into a

__glma_ ---d-h5

I+--

FIN 1.Experimental sppaatur for ton dip spectroscopym a .wpcrsomcfrecJct. L. Icns,M- mrror, B.S.* beam splutcr, F filter, G: gratmg, SHG. KDP crystal, P.M.* photomullip~cr, CM: channelmultrpher, C.A: current ampht3r. OSC- osolloscopcand Rcc: rccordcr. 401

KDP

crystal

to generate

was used as fhe pump

a second harmonic hght. 80% of

(01)

the output

which pumps

a dye laser (Molcctron DL 24, fwhm Q 1 cm-l) which WJS turned into a second harmonic q (probe light).

The pump and probe bghts were focused by lenses of f= 100 mm andf= 35 mm, respectively, and were introduced

into a vacuum chamber.

chagonally

at the center

to cxcitc the cold portlon probe

They cross each other

of a supersomc of thelet.

free Jet m order

The pump

hghts overlap for several nanoseconds.

of the apparatus of the Jet and the detectIon valved are the same as dcscnbed previously

and

between vaned

the from

pressure

sion easily occurs between le, u) and 1g.u) levels after pumping

from lg,O) to le, IJ). In order to maximize the ratlo of the depth of an ion dip to the background ion current, the pump laser (wt) intensity was taken ~30 tunes weaker than that of the probe laser (~2). There-

fore q

system

S, and w2 to both ionization and stimulated emission from S,. In that case, ion signal is observed only when w2 is introduced (see fig. 2~).

m-

5 to 20 mm In order

pomt

(x)

was

3. Results and discussion

to get vaTIous cooling

conditions. The original Idea of Ion drp spectroscopy unplicitly assumes that the energy separation between the real intermedIate state le,v) and the ionization continuum IS larger than that between le, u) and vlbrattonal levels of the ground state Ig, LJ)(fig. 2a). This is actually the case for a diatomic molecule such as 12. However, in large polyatomic molecules such as toluene and anihne, the former energy separation is smaller than the latter by ~0.5 eV. Therefore, even when stimulated emrssion occurs between le, u> and the lower vibrational levels of the ground state, an ion dip is obscured by the sunultaneous MPI from thermally populated vibrational levels

(b)

In fig. 3 is shown the part of the observed Ion dip spectra of toluene and aniline m a supersomc free jet. Both spectra were obtained by exciting the O-O transition of the fist excited singlet state (mr*). Then, the

transitIons were stimulated from the vibrationless state of Sl (ISI ,O)) to the vibrational levels in the ground state (ISu,u)). which were observed as ion dips as shown in fig. 3. For both molecules several vibronic bands were observed which are exactly the same bands appearing in the SVL fluorescence spectra from the IS,, 0) level (see table 1). The spectra were taken under such conditions that the stagnation pressure was ~2 atm and the distance between the nozzle exit and the exciting point

(C)

z 0

-y1

ROOM

TEMP

IOLUENE

ANILINE

00 EXCITAIION

00 EXClTAllON

SSFJ

rig. 2. Ion dip spectroscoples Tar(a) IP - E, u > E, u - E @) tP - Ee,, < Ee,” - Es u at room tcmperbrc. ahd (c)v -%,,
402

contributes only to the pumping from So to

was =:3_ atm and the distance

nozzle exit and the excitmg

I.31

(see fig. 2b). One way to remove such a difficulty IS the method developed by Cooper et al. [2]. The other is the utlhzatlon of a supersonic free jet which enables depopulation of Ig, II) levels. Hence, population inver-

The details

[3,4] Throughout the experunent He was used as a carrier gas and the diameter of the nozzle exit (0) was 400 m.

The stagnation

24 Scptcmbcr 1982

CHEMICALPHYSICS LETTERS

Volu~ns 91. nunlbcr 5

Fg. 3. Ion dip spectra of toluenc (left) and anitme (nght) in 3 supersonic f%C JCt. Both spectra were t&cn by exciting tic O-O band of S, (me). The ordinate IS III arbhry umts.

CHEhllCAL

Volume 91, numbcr 5

I Ion dip inlcwtics

24 Scplemtw

PHYSICS LETTERS

1982

Table

SW.

l~/Ig, lolucnc and andme

fluorcsccn~r:

Asagnmcnt

mlcnswcs IF and wkuhtcd

3, AF (cm-‘)

(f&)x

ton dip mtcnslllcs ~‘1/(o’, + 02) of the wbromc bands uf

100

I,-@-0

100) b,

k7;/0; + 02) x 100 02 = 1.3 oy-”

lolucnc

6ay

525

I8

23

15

6bf

67-l

28

51

28

I0I

793

32

65

33

RIO

23

30

19

428

25

13

q nndmc

1; 63: 1y1;

53.5

24

50

24

1252

21

60

27

1763;

1355

I2

23

13

12p1;

1433

17

34

I7

6&,0

1481

I4

25

14

9ayr;

1713

18

35

I8

rrom rcfs [S 1 (Iolucnc) and [6] (andmc)

= I 6 0y-O

31

b, Ref. 161.

was =20 mm. The fwhrn of each vibromc band is =I cm-l showing that the rotational temperature is under 10 K. The reduction of the ion current was 20-30s (peak height) of the background for each band. However, it was found that at larger X/D we have a larger ion dip. Since X/D is a good measure of rotational cooling, the observed phenomenon is understood as follows. The depth and the width of an ion drp reflect the rotational temperature of the molecules pumped to the St state. As rotational cooling proceeds, the population III the S, state is concentrated around the origin of the O-O band. Therefore the strmulated transition from (St ,O) to ]So,u) occurs in the very hnuted energy region, whrch makes an ion dip deeper and sharper relative to the background. From the above drscussion, it is sug gested that the utihzation of a supersonic jet is quite effective in observing a weak ion dip signal. Table 1 shows the observed ion dip intensities of indtvrdual bands I& (defined as the ratio of the peak height of an ion drp fD, to the background In) and also corresponding relative tntensities of the SVL fluorcscence spectra IFI reported by Smalley et al. [5,6]. As is seen from table 1, those bands that appear strong in the fluorescence spectra also have large intensities in the ion dip spectra. However, the intensity distrlbutlon

is less pronounced m the Ion drp spectra than m the SVL fluorescence spectra. In order to elucrdatc this dtfferencc, we apply a kinetic schcmc to the system shown in fig. 2c and have the followmg differential equations dNo/dt = -No/t uy - ‘, dN,idt =No(t)l,o~-o dh$ldt = N,

(1) -N,(t)~1gi,

t/p,

+y), (2)

W,o, ” _,

(3)

,

(4)

dN,/dr = Nl(t)f,o;

where No, Nt , N2 and N3 arc the populatrons of So, St, the ionlzatron continuum and the vrbrational level of SO, rcspectrvely. y is the rate of all the intramolccufar dissrpating recesses of Sl that arc not mduccd by photons. of-’ and 9 are the cross sectron of the O-O transition between So and Sl, and that of the transrtron between the vrbrationless state of Sj and the lonm tion continuum, respectrvely. o’, represents the cross section of the transition from the vrbratronless state of St to a vrbrational level in So. Assuming steady-state conditions for St, the ion dip intensity ID//n is written as

403

Volume

CHEhllCAL

91, number 5

( :2p,l, __ +,,($g-‘.

2L I,(

4_l _

PHYSICS

(5)

iB - 1.p;

4

__ o,,z .

f I,o,

=

Hence I,//, is Independent of 1, m thus case. In order to confirm that ID/I, is independent of f2, I2 dependences of the total ion current Ig, and I&g were measured for the Iq band of anihne, which is shown in fig. 4. As is clearly se& from fig. 4, f, is proporttonal to II_ indtcatmg that the probe laser is responstble for the one-photon iomzatton from S, and there occurs no saturation m the process. Ftg. 4 also shows that ID/I, ISindependent of 12 withm the laser intenstty range studied. Furthermore, the results for in abd ID/IB indicate that ID is also proportional to 12 and saturation effect is untmportant in the JS,,O)+ JSu,u) transition.

PROBE

LASER

FIN.4. 0’ totnl Ion CUrrCfll, cmisaon)

IB,

INTENSITY

(wlthout

Ofmlmc

versus probe bscr mtcnslty.

f2_

stimuhtcd

ion dip mtcnsity, I&) X 100, of the 1: band of amhnc versus12. Both absclsu and ordmalc are m arbitrary umts.

404

l

1962

It ISclear from eq. (6) that the ion drp intensity ID/IB is not proportional to the band intensity in a SVL fluorescence spectrum wluch is governed by urt. However, f~/f~ can be calculated through eq. (6) by use of Firstly, we postulate the relation between of-’ and 02 as u2 = solO-O. This postulate has the assumption that a2 varies little as the probe laser energy w2 changes. This 1sjustified by the observation that the h4PI spectra of benzene and its denvatives (resonant with St) faithfully reflect the true absorption intensity between So and St and are little affected by u2 [3,4,9]. Secondly, we have a relation between cry-’ and art as CT;= @-“. /.Iis obtained from the intensity of the vibronic band in the SVL fluorescence spectra relative to the intensity of the O-O band [5,6]. The value ID/It, thus can be written in terms of a! and /I as

u;.

hfetimes of the St states of toluene and anilk are M4 ns 171 and 7.6 ns [S] , respecttvely, and therefore the magmtudes of r are of the order of lo7 and 108 s-l, respectively. On the other hand, the focused probe laser tntcnstty r2 IS~5 X 10z5 photons/cm2 s, which makes /20 to be on the order of log photons/s. We can therefore neglect r in eq. (4) and have The

!E_ bJ; -

24 Scptcmbcr

LETTERS

*

po”-O I

‘II -= *B

@J;-”

++0

=J-P+a.

(7)

Therefore, once a suitable value is chosen for a, the ion dip intensities ID/IB for individual bands must be explained through eq. (7) hy using f3obtained from the SVL fluorescence spectrum. The value of 01was estimates for some benzenes by a couple of authors [9,10], but their values cannot explain our experimental data. ‘Ihe choices of a = 13 for toluene and cr = I .6 for aniline explain well the observed ID/Ig values as is shown in table 1. me above drscusaron demonstrates welt that an ion dip spectrum directly gives the ratio of the transition probabilities u1O-O (from the ground state to a resonant intermediate state) and 02 (from the intermediate state to the ionizatton continuum). For diminishing the effect of 7, a probe laser with higher intensity should be used in such a way that no saturation occurs between jg,u) and ]e,u> or ]e,u)and the ionization continuum. The result that u2 is a little larger than cry-’ for both toluene and aniline is not consistent with the work by Boesl et al. [IO]. They report that al-’ is larger than u2 for the molecules studied except for toluene. We consider our result to be more probable because it is denved directly from the intensities of the ion dip spectra and because the character of the resonant intermedtate state (St) does not differ so much between the two molecules.

CHEMICAL

Volume 91, number 5

PHYSICS

151 J.B. Hopkms.

References

[a]

111D.H.

Parka,

J.O. Berg and hl.A. El-Saycd,

m laser chcmlstry,

m Advances

cd. A H. Zcwrol (Sprmgcr,

p.320. 121D E Cooper, C hl. Kbmsck

Bcrbn,

1978)

and J W. Wcsscl, Phys. Rev.

Lettcrs46 (1981) 324. K. Kaya and hl. Ito, J. Chcm. Phys. 72 (1980) 131 J. hlunkaml, 3263. 141 J. hlurakamr, h!. Ito and K. Kaya, Chcm. Phys. Lcttcrs 80 (1981)

203.

24 Scptcmbcr

LETTERS

D E. Powers. S hlukdmcland

1982

R.E. Smalley

I. Chcm. Phys 72 (1980) 5049. D.E. Powers. J.B. Hopkmsand R.E Smalley, J. Chcm. Phys. 72 (1980)

171 G hf. Brcncr, P.A Trims l-x&y [S] D.A. Chcrnoff 2521.

5721. Hackclt,

D. Phdhpsand

Sot. 68 (I 972) 1995. and S.A. RICC. J.Chem.

(91 J.H. Brophy and C.T. Rctlncr, (1979) 351.

hl G Rockcrly

Phys. 70 (1979)

Chcm. Phys. Lettcrs67

[ IO] U.

Uocsl, H.J. Ncusscr and E.W. Schlag. Chcm. Phys. Lcltcrs5.5 (1981) 193.

405