Emission spectra of supersonically cooled methyldiacetylene cations

Chemical Physics 00 (1983) 151-166 North-Holland PublishingCompany

EMISSION

SPECTRA

Samuel LEUTWYLER, Physikalisch

-Chentisches

151

OF SUPERSONICALLY

Dieter KLAPSTEIN

IasGrur der Unicersitiiz

COOLED

METHYLDIACETYLENE

CATIONS

and John P. MAIER

Basel. KIingelbergsrrasse

PO. CH - 4056 Base/. .%ir:erland

Received 10 February 1983

The i2E -S % ‘E (X= + l/Z, - l/2) electronic transitions of rotationally/vibrationalIy cooled CH, --C=C-CIC-H’ cation, as well as the d,-/d,-/d,-substituted species, were studied by emission spectroscopy. Ion emission was obtained by electron impact on the neutral species seeded in a helium supersonic free jet. Vibrational frequencies in both electronic states arc inferred to within + I cm-‘. t splittings are observed and interpreted on the basis of non-linear vibronic Spin-o rb’t couplings. Rotational subbands are observed. yielding rotationai and Coriolis parameters as well as rotational temperatures.

1. Introduction

CD,-C=C-EC-D-

The methyl-substituted diacetylene cations are important prototypes for the large class of substitgted di- and poly-acetylenic open-shell cations which have been found to relax radiatively from their first excited doublet states [1,2]. The relaxation pattern of all the simple alkyl-substituted diacetylene cations is unusual in that the excited cation& A states decay radiatively [3] and yet also lead to fragment ions via non-radiative processes [4]_ The fluorescence quantum yields of the different H, D-isotopic pentadiyne cations were recently measured by the photoelectron-photon coincidence technique and found to be slightly less than unity in the A state, with slowly decreasing quantum yields towards higher internal energies 151. In the present study the emission spectra of the 1,3-pentadiyne cations were obtained by electronimpact ionization of the neutral precursors seeded in a helium supersonic beam [6]. By supersonic cooling rotational temperatures T,,, I: 6 K and T,.ib=G50 K were achieved, resulting in a greatly improved resolution of the rovibronic spectral structure. The species studied were CH,-C=C--C=C-H’

(h,-PD’),

CH,-C=C-C=C-D+

(d,-PD+),

CD,--C-C-C=C-H+

(&PD+),

0301-0104/83/0000-0000/$03.00

0 North-Holland

(d,-PD-).

The optical transitions studied for all four species are between the A’E excited and A ‘E ground states (under assumed C,,. symmetry). The present vibrational analysis is. with a few exceptions. in good agreement with the previous spectroscopic work which vi-as performed on roomtemperature samples. both by laser-excited fluorescence [9] and by electron impact on a pure. thermal. effusive beam [3]. Vibrational frequencies in both the ground and excited states can now be inferred to much higher accuracy (i 1 cm-’ or better) due to the narrowing of the bands and improved resolution. Emission from excited vibrational levels within the A state was observed up to internal energies of = 1800 cm- ’ above the vibrationless state. In a number of instances Fermi resonances were observed between totally symmetric (al)

modes

and also between

totally

symmetric

and degenerate (e) modes (sections 3.2 and 3.3). Of special interest is also an improved analysis of the degenerate e modes which are capable of inducing Jahn-Teller distortions in both the ground and excited electronic states (section 3.3). A partial resolution of rotational structure was also achieved (section 3.5). leading to additional information on Coriolis couphng parameters. spin-orbit coupling coefficients and rotational temperatures.

bandpass

2. Experimental

nm \vere calibrated with a neonlamp. The samples of 1.3-pentadiyne. I-deutero-1.3pentadiyne. 5.5.5trideutero-1,3-pentadiyne and tetradeutero-l.3-pentadiyne were synthesized as described in the literature [7.S] and purified by gas chromatography. filled

The emission spectra were recorded with a crossed electron-beam-sample-beam apparatus. incorporating a supersonic free jet. which has been described previously [6]. In brief. the samples. at vapour pressures of lo-30 mbar. were expanded with helium at total backing pressures of 0.4-l-4 bar through a 70 pm diameter nozzle into the ionization chamber. where the pressure was -C lo-’ mbar. Excitation was accompiishod with a collimated = 200 eV electron beam. 3-6 111.4 current. which intersected the free jet = 5 mm downstream of the nozzle. Photon emission was monitored with an J/9.5. 1.26 m monochromator and single photon counting electronics. and recorded digitally on-line with an LSI- 1 l/O3 microcompurrr. The numerous helium emission lines. obtained simultaneously. were used for wavelength calibration and determination of the optical resolution. \vhich was 0.032 nm for the full spectra (figs. 1 and 2). Higher-resolution

recordings

of selected

18000

17500 Fig. 1. The A’E -+ Z? ‘E (X= impact

fwhm.

on

P seeded

helium

i-1/2.

supersonic

- l/2) free

jet.

bands at a

of 0.01

thorium

hollow* cathode

3. Results and discussion

The electron-impact-excited emission spectra were measured for all four isotopic 1.3-pentadiynes in the energy range 15000-23000 cm-‘. Band

systems of the

of 111~3 ,&‘E + A ‘E elecrronic corresponding radical cations

rransitions were ob-

served. The spectra of II,-PD+ and d3-PD’ are sho\vn in figs. 1 and 2. As has been shown by Heilbronner et al. [lo]. the two lowest electronic states are dominantly

18500

described

by the MO config-

19000

emission band spec~rurn of CH,-CFC-CEC-H+ e.xcid by = 200 s\J rlectron Helium emission lines arr marked with 1 dot. The op&z4 bandpass \~as0.03~ nm

I

’

’

’

18000

17500 Fig. 2. The i\‘E

+ % ‘E

(‘=

-k l/2.

- l/2)

18500

19500

19000

emission band spectrum of CD,-CGC-CEC-

H-

r.xired by = 200 SV slcc~ron

impact on a seeded helium supersonic free jet. at an optical bandpass of 0.032 nm fwhm. Helium Smithson line> xe nurksd with a dot.

urations (le)‘(l la,)‘(2e)“(3e)3 (for A;<>and (1ej-l (1 la1)‘(2e)‘(3e)’ (for A)_ The 3e molecular orbital is expected to be antibonding with respect to the C-C single bond and bonding with respect to the C=C bonds, whereas the 2e molecular orbital is bonding throughout. One may then expect progressions in those vibrations that involve displacements in the central single bond and the acetylenic bonds (in the internal valence coordinate description).

These

are the normal modes v;(Y, C=C,). and I~,(v> v‘I(vs C=CH(D) ), Q(V~ C-ChC-C) C-C=C-C), using the normal-mode analysis of Lamotte et al. [8] for the neutral prntadiynes. These modes indeed represent the main source of vibrational activity in this electronic transition. as can be seen in figs. 1 and 2. The symmetry is assumed to be C,,. in both electronic states: this is consistent with the observed vibrational intensity patterns (weak intensity of the degenerate bending modes). and lvith the rotational structure observed (parallel bands. indicating pure r-axis polarisation of the electronic transition dipole moment, see belolv). The photoelectron spectra of the 1,3-pentadiynes also indicate that the linear configuration of the carbon

is retained in borh *q and .A srares. based on the intense adiabatic transitions of the first two bands [ 101. Both the k and .< states are slscrronically dr3generate and thus susceptible to distortion along any or several of the degenerate e-type bending normal modes. due IO Jahn-Teller coupling. Although the lack of intensity of these bending modes indicates fairly NIX& Jahn-Teller coupling. it is interesting that additional structures have now been resolved in bands involving the rt13 mode lvhich are attributable to a dynamic Jahn-Teller effect in the ,% -E state. atoms

Due to the narrowness of the rotational band structures and the reduction of the sequence band structure by the supersonic cooling. the accuracv has heen considerably improved. The maxima if the electronic origins are given in table 1 (optical band-pass 0.01 run) and lie \vithin the +4 cm-’ uncertainty of the values attained by laser-excited fluorescence spectroscopy on room-temperature samples [9). Spectral shifts of the O,$’band seem 10

154

S.

~~~twyler

et aL /

Emission

Table 1 Maxima of the elecwonic origins (0,” bands) of the A*E + 2 *E

emissionsystems(cm-‘)

CH,-(C&),--H+ CH,-(C=C), CD,-(C=C),-H+ CD,-(C=C),-D’

-D+

Present work

LEF [9]

20374.5(f OS) 2039l.q & 0.5) 20374.7(f0.5) 2039O.q * 0.5)

20374(+ 4) 20378(+- 4) -

be correlated with deuteration at the terminal hydrogen position, but not with deuteration at the methyl hydrogen positions. Most of the vibronic transitions observed correspond to transitions between the vibrationless excited state and groundstate totally symmetric fundamental modes and overtones or combinations thereof. The proposed assignments are indicated in figs. 1 and 2 and gathered in table 2 for all four isotopic species. The fundamental frequencies derived are collected in table 3. Emission is also observed from excited vibrational levels of the electronically excited A’E state. These levels are populated by the electron-impact process A2E + X IA, according to the Franck-Condon factors for the transition from the molecular ground state to the ionic excited state. These are essentially the same Franck-Condon factors as for the photoionisation process which can be obtained from the photoelectron spectrum. Of the seven a,-type totally symmetric fundamental frequencies, three were obtained for the A state and five for the 2 state (see table 3). Of the vibronic transitions observed, the most strongly developed are the 7: and 6’: bands (as well as their excited-state counterparts, the 7; and 6: bands), in all four of the isotopic pentadiyne cations studied. The normal modes involved are approximately described as C-C single bond stretching motions [f&l 11. The potential-energy distribution terms calculated for neutral h,-pentadiyne give about equal contributions from both the H&--C bond stretching and the central C-C bond stretching coordinates for both of these normal modes [l 11.For both the V, and Y, vibrations two members of the ground-state progression are

spectra

of methyldiacetykne

cations

observed. Also clearly observable are the 3: and 3: bands; the corresponding normal mode is dominated by the C,rC acetylenic bond-stretching coordinate, with a small admixture of the central single C-C bond-stretching coordinate in the potential-energy distribution [ 111. In the laser excitation spectra of both h,-PD+ and d,-PD’ transitions to the vi (A2E) levels are only weakly observed. Conversely, the 4: emission band is also expected to be weak. The normal-mode analysis of 1,3_pentadiynes shows that v_,is essentially described as the -eC,stretching motion. Deuteration at the terminal hydrogen position leads to a slight decrease of v~: neutral h,-PD and h,-PD exhibit u4 = 2072 cm-’ and 2071 cm-‘, respectively, which decreases to 1961 and 1958 cm-’ for d,-PD and d,-PD, respectively. This characteristic shift allows an unequivocal identification of the weak 4: bands for the corresponding cm-’ for h,cations: UT = 1921 cm-‘/1920 PD+/&-PD+ and 1882 cm-‘/1884 cm-’ for d,respectively (see figs. 1 and 2 and PD+/d,PD+, tables 2 and 3). An alternative assignment for these weak, but distinct, bands is as 1360,which we reject as the intensity of this high overtone transition should be negligible, and also because splittings due to spin-orbit and Jahn-Teller coupling are generally observed on the degenerate overtone bands such as 13$?, 13: and 7:13,0_

The 5: band is well-developed in the case of ds-PD+ (see fig. 2) and d,-PD+. It is not clearly discernible in the h,-PD+ and dr-PD+ emission spectra, where it extensively overlaps the 6: band and 7:13! combination bands. Obviously this set of levels lying in close energetic proximity is strongly mixed by Fermi resonance and therefore the identification of the 5: band is problematic for h,-PD+ and d,-PD+. Full deuteration of the methyl group shifts v;’ from 1260-1280 cm-’ down to = 1000 cm-‘; this characteristic shift, which is also exhibited for the neutral molecules, is the prime criterion for assignment of the corresponding vibronic transitions in the spectra of d,-PD+ and d,-PD+. The 5: bands of the deuteromethyl species exhibit a fairly complex structure, being a group of four or five subbands spaced irregularly over an interval of = 100 cm-‘. This structure is due to a perturbation between v;’ and an as yet

.S. L.eurwyfer ef al. / Emission specrra of rnerhyldiacerylene

carions

155

Table 2 Maxima

of the observed

are numbered Assignment 6;7,:

bands (cm-‘) as in table 3. Tentative CH,--(C=(Z),-H’

i 6a13;

22162.7 22106.5

7;13;

21643.3

6;

21503.6

in the A2E - % 2E emission spectra of the 1.3-pentadiyne cations. Vibrational fundamentals assignments are in parentheses. All values + 1 cm-‘, except for the 02 bands (50.5 cm-‘) CH,-_(C=C),-D’

21639.5 21616.5 21512.3

21594.0

($/l2’,) 7: 13; 6;13,” 6’7’ $7’0 0 i 7;13; 0,” 2. (13:) 7’6’ 0 I 13: 7P 7; 6: 13:

7p13,o 7:

(5W2D,)

6;13;

CD,-(C=C),-H’

21536.5 21425.6

CD3-(C=C)2-D+

21547.3

2 1409.0

21045.9/037.6

21042.9

21403.6 21356.0 21001.6

.20985.2/978.7

20987.0

20947.1

20957.6

{ 20958.2/951.8 20875.5

( 20961.7 20903.9

20856.2

20882.3

20815.1

20834.S

20891.1

20912.0

20709.9 20443.9

20773.6

21568.6

20447.8 20417.2 20397.2 20374.5 20346.3 20298.9 20102.7 19834.2 19778.2 19747.4 19725.6 19688.8/683.1 19667.6 19170.7 19139.9 19126.2 19077.5 ? 19077.5 ? 19047.5 19028.6 18994.4/983.3

19095.5

18574.1 18541.0 18520.4 18494.4 18453.4 18162.0 18141.8 17965.8

21373.6 21006.9

20418.9 20391.4

20397.8 20374.7

20367.3

20354.0

20327.0

20311.6

20136.2

20066.9

20390.0

20142.S 19789.7

19841.9 19805.3

19816.4

19837.8

19783.2

19789.7 19770.9 19731.9/727.7

19823.7

19685.7

19708.5

19740.1

19208.6 19221.4

19152.3

19171.2

19762.5 19714.2

19805.3 1975S.S

19208.6 19168.3 19139.4 19111.6 19095.0 19074.6 19034.9/28.2

19139.4/130.6

19133.8

19171.’

19115.2 19082.2 19368.0 19346.0 19333.5 19324.1 19307.7

19155.0

18565.4

19118.4/111.6 19356.4 19369.8 19351.8

19334.4 19321.4

18581.0

18576.4

18559.9

18557.1

18515.0

18517.2

18523.2

18508.6

18454.3 18162.8

18506.3 18203.9

18204.3 18183.5 18007.1

18144.3

Tuhle 2 (continued) CH,-(CZ&)2

CH,-(C=C~.--H’

Assignment

6’7’13: I I

P

17859.7

Q

17834.6

CD,-(C=QI--H‘

-D’

Cq,

-(C=C)2

6’7’ I z

17607.9 17578.8

17627.3

3$?3;

17540.9 17520.0

17602.5 17581.0

17554.7 17565.7

17642.5 17621.9

3y7p

17520.2

17572.6

17477.1

17531.7

3:6:

16973.6

17033.3 17014.s

3:

15966.4

16034.0

16956.3

unidentified second vibration. The two main possibilities are: (1) A Fermi resonance betxveen the degenerate overtone 2~;; and Y;‘_ The v;i vibration is a skeletal-bending mode, with a fundamental frequency in neutral d,-PD and ti,-PD of 475-485 cm- ’ [8]. Homogeneous perturbation (Fermi resonance) is possible between the totally symmetric A, component of 2~‘,‘, and v;’ (A,), or, if vibronic coupling is important for the v,? mode. between

Table 3 Fundamental

vibrational frequencies

states from the laser excitation

(i:

1 cm-

‘)

of the h,-PD+.

the E vibronic components

the vibrational levels lie energetically close to each other. the interaction is expected to be quite strong,

d,-PD’.

spectra 191are given in parentheses.

Species

State

CH,-(CrC),-H

X

CH,-(C.=C),-H’

2 2;

‘A

y3

“4

2257 3)

(2072

221’

1921

d,-PD‘

CH,-_(C=C)2-D CH,-(C=C),-D-

X ‘A R2;

2156

(2000) 1961

and d,-PD’

radical cations in the ground.

neutral molecules are taken from ref. [S]. Values for

Values for both spin-orbit

components

“5

1.7

v6

1375

A\‘E (2135) 2247 ”

of 2~;; and 18;’ (whose

vibronic symmetry is E). (2) Heterogeneous perturbation (Coriolis-type perturbation) of v;’ by the E(j= l/2) vibronic component of the degenerate UC mode by a dl= f 1 coupling mechanism. The vS mode is the symmetric -CD, bending mode and vg is the corresponding antisymmetric -CD, bending mode; as

states. Thr values of thr corresponding

and first excited, A2 E. electronic

---DA

(1272) 1377

1882

A’E

are given for Y, and vrs VI3

Cl

1152

6S6

320

1203

685/691 b’

313/324h’

1130

664/672 h’

(1131) 1146

(666) 676

303/306 h’ (306) 314

1196

676 h’

_ 303/314h’

1122

652 ”

29s h)

CD,-_(C=C)a--H

X’A,

2252

207 I

1023

1182

652

308

CD,-(C=C)a-H+

%(‘E

2212

1920

1036 b.J’

1222

643/647 h’

292/302 h’

(1991) 1958 1884

A’E

982 b’

CD,-(C=C),-D

Xl;\,

(2182) 2239

CD, -(C=C)2-D+

X’E

21S6

.i’E nr Corrected for Fermi resonance. b Affected by Fermi resonance. not

c, Deduced from 2~~3. d, Strongly mixed with 2v,,

corrected.

or v, (see text).

(977) 1021

1162 (1158)

627 h’ (622)

% ‘E.

the A’E

2S6 h’ (284)

1180

646

298

1020 h.d)

1219

634 h1

283/292 h,

1019 b’

1157

617”

284 hr

even for this higher-order perturbation mechanism. A sinlilitr situation arises for both (I,-PD’ and d,-PD’ in the excited A’E state. where t\vo or three bands are observed in the energy region where the 5: band is expected. We again assign this group of bands as the ~;/21r;~ Fermi diad or to 11;perturbed by Coriolis coupling to 19;. In the analogous electronic transitions of the 2.4-hexadiyne cations the corresponding vibronic transitions are not observed at all. Nevertheless. the vibronic assignments just discussed_ and subsidiary information derived from the K-type rotational structure 8s analyzed below_ indicate that the methyl group undergoes a slight geometric change upon electronic excitation. The observation of the (&.,CH,) deformation modes and the apparent absence of the (v, ,CH,) stretching modes indicate a change of the methyl-group apical angle on electronic excitation. Table 3 compares the vibrational frequencies of

the neutral pentadiynes (X ‘A, ground state [S]) with the vibrational frequencies of the corresponding radical cations. both for the ground ,% ‘E and excited A’E states. Several characteristic frequency changes are apparent. which are in good accord with the simple MO configurational scheme outlined above. Removal of an electron from the highest occupied 32 orbital leads to an incre:t>e in the single-bond-stretching frequency z?, and to dscreases in the C-C bond-stretching frequencies api and V, of the ionic ,% ‘E ground state. as compared to the neutral X ‘A, ground state. Removal of an electron from the 2e orbital generates the ionic ,&‘E state. with frequency decreases no\v also in the single-bond-stretching frequencies zk and ri_ as well as for V> and Y.,. 3.3. Jahrz- Teiler effecr ad In the pentadiyne

degtwerure

e rmries

cations. both the ground state X ‘E and the excited state A’E are doubly degenerate and subject to a Jahn-Teller distortion by the interaction of degenerate vibrations of e syncmetry with the electronic wavefunction. Although previous spectroscopic evidence indicates that the Jahn-Teller effect is not overly important for these species. we find it necessary to include vibronic

coupling

for the discussion of the finr structure of vibronic bank and also of sequence hands obser\:thlr under the nnxv ixnprovcd spectral conditions. Following the treatment of Longuet-Higgins et al. [ 1?.13] we brisfl_v review the expected vibronic

drgenerate

level structure for the wi’ak_ linear coupling case. The term snerpics resulting for an c-symmetry

\9-mtion with unperturbed frequencv tional quantum number L’are: G(u./)=G’(L.f

1)T2Dw(l;

w and ribra-

!).

(1)

where I is the vlhratinnal angular ~nonientuni cluantutn number md 1) is the Jahn-Teller coupling pnrmnctsr. For csample. for 1’= 2. I can be 0 or 2. and a zplitring takes place into equidistant levels wi iii vibronic s> mmetries (.A !_ I __j = 3,1’2)_ E( j = I /?.I and E( j = 5,/Z) and energy separations of ~DLG bsrxvssn lsvcl~ (fie_ 3). Further splitting of the A,. .A, pair of levels due to quadratic coupling is neglected. The quantum number; distinguishes rhe differen: Is\~ls of a given u and takes the values I = l/Z (I + 1,/I! for I = 0).

We fttrthermore nrcd to consider the rota1 intsrnal vibronic angular rnonlentum around the svnlnperametric-top axis [ 1_7.14]_ Th e corresponding tar I)_ is the sum of an electron-orbital contribution L_ and ;I vibrational contribution G.. The vibronic (L,)

esprctstion

value of L, is given b>

=
(2)

expectation value of the operator. The ‘-quen&ing paranwri ri depends on the Jahn-Teller distortion parm-nmx D. \vhere II= 1 in rile xvealr-coupling limit and tends to zero in 111~ srrung-coupling limit. thus qucnchInclusion of quadrstic ing ST, ~12 D incrsrxsss. Jahn-Teller coupling terms accentuates the \vhcre ;, is the electronic L,

quenching process. The \-ibronic expectation ~nluc of G_(r) single Jrthn-Teller active normal mode r-is (G,(r))

= .;,(i-

t//Z).

for ;I

(3)

The expectation value of the total vibrottic gular nionientuni is given by [Iq]:

an-

(~=>=s’,,=~~‘,ii(~~--~~)(~ic~,.1,..

(4)

I’

158

S. Leuhcyler et al. /

(b)

(a)

w

Emission spectra of methyldiacetyfene cations

W

0

Fig. 3. Vibronic transitions for a degenerate e-type mode in an E * E w.tnsition: (a) vibrational stales. no vibronic coupling, (b) Jalm-Teller coupling included. (c) Fermi resonance with v;’ (vibronic symmetry e). (d) scheme of resulting emission speetrum including vibronic coupling. Only parallel bands are shown. Not included are spin-orbit splitting and I-type doubiing effects. Note that the higher energy 0 + 2 transition (dashed line) only becomes allowed when non-linear coupling terms are included.

The sum term runs over all degenerate normal modes apart from the Jahn-Teller active distortion mode. Thus the effective Coriolis parameter S;” of a vibronic sublevel depends on its “rotational quantum number” j as well as on the quenching parameter d, and affects the rotational K-dependent structure [see eq. (11) below). The electron-orbital contribution Ied influences the spin-orbit splitting directly [see eq. (7) below]. Thus the spin-orbit splitting of degenerate vibrational states which exhibit Jahn-Teller coupling is expected to vary markedly from the value which is typical for the totally symmetric vibrational states with unquenched {,. For the weak, linear coupling case the sublevels of a given Jahn-Teller active vibrational state may only combine by optical transitions according to the selection rule for vibronic angular momentum [12]: J-‘I -

I’=

_e

l/2.

(5)

Hence from the A vibrationless state (I’ = 0) only the E(j” = l/2) sublevel of 2~;; is accessible by a dipole transition (cf. fig. 3). It would be extremely interesting to obtain theoretical estimates of the Jahn-Teller parameter fok the type of molecule considered here. The only work we are aware of is an ab initio calculation on the ZE ground state of methoxy radical (CH,O) by Yarkony et al. [15]. They find a very small minimum in the bending coordinate: for the C-O bond off the symmetry axis by 2.5O, the total energy is = 70 cm-’ less than for C,, symmetry. As previous work has shown [3,9], at least one of the non-totally symmetric degenerate e-type modes is observed in double-quantum excitations, both in the excited and in the ground state of the pentadiyne cations. Similar features are also observed in the A*lI + % *II electronic transitions of the halo-substituted diacetylene cations [2], in the A*E --, % *E transitions of the 2&hexadiyne cations [5], and also in the A*II, --, 8 *IIg electronic transition of diacetylene cation itself [ 161.In the 1,3-pentadiyne cations (figs. 1 and 2) one of the degenerate modes appears in close proximity to the 7,-j/7: bands, some 50-80 cm-’ closer to the electronic origin. The first-overtone frequencies observed are not severely influenced by deuteration, either at the methyl or at the terminal hydrogen position, but shift approximately parallel to the Y, fundamental frequencies. Hence the degenerate mode is one of the skeletal bonding modes VII -Y,,,, and is assigned as 2~~~ on the basis of the neutral molecular frequencies [8,1 I]. The intensity of the y13 overtone derives from Fermi resonance with the Y, mode, similar to the case of diacetylene cation [17] and the diacetylene cation derivatives mentioned above. The 13: bands occur as a group of at least three subbands in the emission spectra of all four isotopic pentadiyne cations: we denote these subbands by a, /3 and y in order of increasing energetic distance from the 7: band (see figs. 1 and 4)_ Table 2 contains the observed transition energies for the a, j3 and y subbands of the 13,0, 7:13,” and 6:13: transitions for all four isotopic species. This characteristic triple subband structure is also observed in the Fermi triad (2+‘, u;’ + 2v;;, 4~;;) and, furthermore, in combination

S. Leurqler

Spin-orbit 8

er al. /

Emission specrra of merh_sldiaceyiene carions

splittirqs in vibronic bonds of CH,- CC-C&-H@ 0

0

We briefly consider the spin-orbit interaction for vibrationless and vibronically excited states. Brown [14] has treated the rotational eigenvalue problem for molecules in ‘E states including a linear Jahn-Teller effect where the electronic degeneracy is resolved by one vibrational mode. In the present case of a near Hund’s case (a) limit, he obtains for the spin-orbit terms: E + = & aSed/

f B,‘/4a~ed

+ {rotational Fig. 4. A comparisonof the effect of spin-orbit splittingon the band structuresof the 0: (a), 3: (b). 7: and 132 (c) vibronic bands. The spin-orbit splitting (in cm-‘) is indicated above the spectra. Helium backing pressure 1.4 bar, optical bandpass 0.014 nm.

with the other a,-type vibrations, e.g. {v: + 2~:~~ V; + v;‘} and {Y;’ +2~;;, v;’ + Y;‘}, as is evident from figs. 1 and 2. Over a stagnation-pressure range pHe = 0.4- 1.4 bar there were no significant intensity changes of the three subbands relative to each other and to the 7: band, indicating that it is not K-dependent rotational structure which is observed. Also, table 2 shows that the frequency separation between the a and p bands is = 20 cm-’ for all of the isotopic species, which definitely excludes K-type rotational structure. By this same argument it is very improbable that the Fermi resonance between V; and 2~;; is responsible for the a/j3 splitting, as slight changes in the unperturbed V, and 2~~~ energies would lead to variations in the splitting. This is supported by the persistence of the 20 cm-’ splitting in the Fermi triad {2~;‘, z$’ + 21$, 4&} and also in the combination bands. The halfwidth of the a and /3bands is less than that of the y band and of all other vibronic bands in the emission system: the fwhm of the cx and j3 bands is 4.3 cm-‘, that of the 0: band 6.0 cm-‘, and that of the 7: band is 10.5 cm- ’ at pHc = 1.4 bar. We conclude that a and /I are the two spin-orbit components of a 2~;; vibronic sublevel which exhibits an unusually small ground-state spin-orbit splitting due to the vibronic angular momentum associated with the level (see below).

159

terms + constant terms}.

(6)

where u is the spin-orbit constant. B, is the usual rotational constant, and 1, and d are the electronic angular momentum component and quenching constant as described above. Thus the vibronic states split into two spin components separated by approximately a&;d, where each component has its associated set of rotational levels (discussed below)_ The splitting of an origin or vibronic band due to spin-orbit coupling is then: Aso = ( a’l;d’

_ a”;;d”)

_

(7)

Thus, to describe the spin-orbit splitting of any given (non-degenerate or degenerate) vibronic band. in general three pairs of parameters are needed: (1) The spin-orbit constants ~1’ and LI”. In the % and A states of the 1,3-pentadiyne cations, the unpaired electron resides largely in the z orbitals of the diacetylene moiety. For the present purposes we hence assume the values of diacetylene cation u’ = - 30.6 cm-’ and u” - - 33.3 cm-’ [16], tb hoe transferrable to 1,3-piiadiyne cation (all isotopic species). In support of this assumption we mention the similar case of the OH and OCH; radicals. The unpaired electron of these radicals is largely localized on the oxygen atom, analogous to the present case, where the unpaired electron resides on the central C-C-SC part of the molecular ion. For the hydroxyl radicals c” = - 139.2 cm- I, which changes by less than 3% to u” = - 143 cm- ’ for the methoxy radical [ 191. (2) The electronic angular momentum components {i and {;’ which are 1.0 for unperturbed pl; orbit& Even for the vibrationless ground states, 5, is slightly reduced from this value by: (a) the perturbing threefold potential intro-

duced by the methyl group, (b) vibronic coupling terms which are off-diagonal in the spin-orbit hamiltonian. (3) The quenching parameters d’ and d”. For vibrationless levels and non-Jahn-Teller-active vibrationally excited levels d’ = d” = f 1 and (7) simplifies to A=’ = (u’{; - n”lZ).

cause 1: is completely quenched for these levels as well as for the 2~;; levels. so that higher vibratiOtld excitation does not decrease the quenching constant any further. i.e. d”(2v,3)= 0. This directly yields the magnitude of the spin-orbit splitting of the vibrationless A state as \a;Sil=

(8)

For the origin band the spin-orbit splitting is small. After a careful comparison of the origin bands of the isotopic pentadiyne cations and a partial rotational analysis (see also section 3.5) we obtain Aso = 1.8 cm-‘. as indicated in fig. 4a. The 3: transition. which leads to a lower-state level which is unperturbed by Fermi resonance with Jahn-Teller-active modes. is also clearly split by Aso(3y) = 1.8 cm-’ (fig. 4b). For Jahn-Teller-active vibronic states. the quenching parameter d decreases towards zero. depending on: (a) the linear Jahn-Teller coupling parameter D. (b) the vibrational quantum number of the Jahn-Teller-active mode, (c) the effects of quadratic coupling terms, (d) contributions by mode coupling in the presence of several Jahn-Teller-active modes. In the case of the 2~;; level, all of these terms presumably contribute to the quenching; hence one expects a very Iow value ford;;. The spin-orbit splitting for the 13: transition is indeed Iarge ( = 20 cm-‘), as mentioned above. It is intriguing that this band and combination bands such as 7:13p and 6:13p, as well as the 13: band observed in Fermi resonance with the 6: band, all exhibit spin-orbit splittings of equal size. falling in a narrow range As0 = 19.5 f 1.5 cm-‘. The 7: band itself exhibits a spin-orbit splitting of As0 = 5.7 cm- ‘, indicating a lowering of the d;’ quenching parameter already for the v;’ state. One therefore expects the 7pl38 band to exhibit a spin-orbit splitting larger than that of the 13: band. For the 13: band, a larger spin-orbit splitting is expected than for the 13: band due to the higher vibrational excitation. We conclude that the spin-orbit splitting of these bands does not increase further be-

19.5 f 1.5 cm-‘.

(9)

Based on the relative intensities of the spin-orbit components of the 13:. 7:13p and 6:13$? bands. we assign the more-intense, lower-energy. bands ((Y bands) as the 2 = + l/2 and the less-intense_ higher-energy. bands (fi bands) as the Z = - l/2 components. Hence the A state is inverted. in agreement with the situation in diacetylene cation. Using the ah value of diacetylene cation as discussed above. we obtain 1: = 0.64 f 0.05. The approximate size of the ground-state value of cl;{: can be derived from the spin-orbit splitting of the tentativeIy assigned 12; band. which is Aso(2v;,) = 22-23 cm-’ (see table 2. d,-PD’ and I?_,-PDi). assuming that ~‘(ZY;,) = 0. An alternative derivation is possible using the spin-orbit splitting of the origin band a;{~=ab<~-AsO(O~)=

-21.3-t_

1_5cm-t.

(10)

in good agreement with the first value. Employing again the a: value of diacetylene cation, we obtain 1; = 0.64 10.05. These IL, c’ values may be compared to that of the methoxy radical, where {y = 0.44 [19]. The replacement of -H by -CH, constitutes a larger perturbation in the case of the hydroxyl radical than in the case of diacetylene cation. The VI; state is quite strongly mixed with the 2~;; vibronic states by Fermi resonance and concurrently acquires an admixture of several characteristics of the latter, i.e. vibrational angular momentum and quenching of the electronic angular momentum to an extent. The observed spin-orbit splitting can be related to the parameters derived above: Aso(7p)

= (&,I; - agIrd;‘)

yielding d”( v,) = 0.65 2 0.07.

= -5.7

cm-‘,

161

Table

.T zE

its closer proximity to {~a;‘> than the lolver lying (j” = 5/2) level. The splitting betlveen the E-type sublevels of {2x*;:;> is = 53 cm~ t. Fig. 3 represents

4

Spin-orbit

paramctrrs

of /I,-pcn~z~diyne

.&‘Esme

state

pCinl”ltWX

vlllus

“0 n, C,, se

- 33.3

SC

cation

pW3Ill&X

cm - I

* 2,

the important

v3lur - 30.6

cm-

0.64 f 0.05

;T

0.64 + 0.05

cI( v;,

1.0

W&)

0.16 f 0.09

cI( z,;, d(Zaq)

0.65 f 0.07

d(2v;j)

- 0.07 * 0.07

d( z*p + v;,, rl( Id’ i t*;;,

0.03 * 0.07 - 0.05 + 0.07

+0.10

’

0.40 f 0.0s

Quenching parameters for the vibrational levels of h,-PD’ for which spin-orbit splitting of the corresponding vibronic transitions was clearly observed have been gathered in table 4 together with associated spin-orbit parameters. The observed quenching of {p in a number of vibronic levels indicates that ground-state Jahn-Teller-type couplings are far from negligible for the ground electronic state. A further indication is the observation

of a third band.

the y band.

and a weaker fourth band which coincides with the He atomic line at 5047.74 A. but is observed on the 6713: combination band, implying that a second sublevel of (2~;;) also gains oscillator strength. The weak, linear coupling formalism does not adequately describe this situation: by the selection rule (5) the only transition allowed from the vibrationless state (I’ = 0) is to the sublevel E(j” = l/2) of {2v;‘,). If the nonlinear and mode coupling terms discussed above are included in the vibronic hamiltonian. states of the same vibronic symmetry (i.e. the E species) are mixed and j ceases to be a good quantum number. Only the sublevels of (2~;;) with E vibronic symmetry species can gain appreciable oscillator strength by Fermi resonance with L$‘. whose vibronic species is E. The other two sublevels of (2~;;) with vibronic symmetry species A,, Al cannot interact vibronitally with the other sublevels or with Y;’ at any coupling strength. Of the two E sublevels, the one characterized byj” = l/2 for weak coupling lies at higher energies for all coupling strengths. gains more intensity by Fermi resonance

and thus through

interactions

schematically.

It is interesting to note that the intensity pattern of xeibronic bands in the 132/7: Fermi diad (relative intensities 35 : 90) change considerably in the nest higher polyad. where the relative intensities of 7ra13p/7t are 15 : 10 (on the same intensity scale). This is similar to the intensity patterns observed for the diacetyiene cation spectrum_ where the totally symmetric x%bration. from which the transition intensity derives. is not the strongest component in the Fermi triad [ 16.171. The 13: band is expected to be by far the lveakest component in the 13~/7,c13~/7~ Fermi triad: in the pentadiyne cations it is overlapped by the 6: and/or 5: bands. Fermi resonances are expected between Vi’_ vy and Jv;>_ Mthough additional structure is clearly observed in the \,icinity of the 6: band. reliable assignments have not yet been possible. , The A state 2v,? overtone also gains intensity by Fermi resonance lvith ~5. A total of four subbands corresponding to the 13: transition are observed to the red of the 7: band. As in the ground state, these are assigned as transitions from the tlvo E-symmetry sublevels of (21*;1)_ each eshibiting spin-orbit splitting. Both the spin-orbit splitting (Aso = 6.5 cm-’ ) and the splitting between the E-type sublevels ( = 27 cm- ‘) are smaller than the corresponding values in the _% state. indicating that vibronic coupling in the escited weaker than in the ground state.

A’E

state

is

As all of the totally symmetric vibrations observed in these measurements and also in the previous laser escitation studies exhibit a slight decrease in frequency (by 3-59) in the .i state. as compared to the X state, sequence transitions are observed to the red of the 0,” band. A number of red-shifted sequence bands of the A, modes have been identified (see figs. 1 and 2. and table 2). Weak. but extensive band structure is also prominent from the 0: band towards higher energies; generally four to six bands vvith irregular spacings of between 20 and 30 cm-’ are observed.

S. Leutwyler

162

et al. / Emission spectra of methyldiocetylene

One or two of the bands exhibit the narrow halfwidths that were previously attributed to separated spin-orbit components. In principle one could expect a perpendicular component (AK = + 1) of the 0: transition to exhibit a K-type rotational structure with band spacings of this magnitude. This possibility can be excluded, however, as the band spacings should be approximately twice as large for the methyl-h, pentadiyne cations as for the deuteromethyl derivatives, which is contrary to observation. The possibility that two or three of the blueshifted bands are due to 7213: transitions cannot be excluded for the h,-PD+ and d,-PD’ radical cations. However, these bands are expected to be quite weak compared to the 7: sequence band, an estimate being possible from the analogous 6~13~ and 6:7p bands. Furthermore, such an assignment can be definitely excluded for the d,-PD+ and d,-PD+ species on the basis of the measured 7: and 13: transition frequencies. We therefore assign the majority of the blueshifted band structure to sequence structure of degenerate vibrational modes. All of the seven degenerate bending modes may exhibit a splitting of the u = 1 level by vibronic coupling into a pair of A levels and an E level. If, as has been shown for the its mode, the ground-state splitting is sizeably larger than the a state splitting, then the A I 2 + 4.2 subbands are blue-shifted and the E +‘E transitions red-shifted relative to the band position without vibronic interaction (fig. 3). The band structure between 20 and 90 cm-’ to the blue of the electronic origin is thus probably due to overlapping of the A,_, + A,., subbands of the 13’, and 14: sequence transitions, together with the 7:13! transitions mentioned above. 3.5. Rotational structure

The B values for the 1,3-pentadiyne cations are less than = 0.07 cm-’ (table 5). Although the rotational lines themselves remain unresolved, the rotational branch envelopes and other fiie structure observed allow valuable conclusions to be drawn concerning: (a) the influence of first-order Coriolis coupling, (b) geometry changes upon excitation,

cations

Table 5 Approximate rotational constants of the pentadiyne cations. calculated for a standard geometry*) (cm- ‘)

/I,,-pentadiyne+ d,-pentadiyne+ d3-pentadiyne+ d,-pentadiyne+

An

En

5.24 5.24 2.62 2.62

0.068 1 0.0646 0.06 18 0.0587

radical

RI Starting from the methyl end of the molecule [22]: T,.-” = 1.095 A, a (HCC)= 109.47”. rC_= = 1.455 A. rCSC = 1.208 A, rC_‘ = 1.375 A, r,--= = 1.208 A. rC_* = 1.056 A.

(c) rotational temperatures of the neutral precursor molecules, (d) spin-orbit splittings, (e) polarisation of the electronic transition. The rotational level structure of the 1,3-pentadiyne cations will be briefly summarized, following the treatments of Brown [14], Hougen [18] and Russel and Radford [19]_ The pentadiyne cations are prolate symmetric tops; approximate rotational constants were calculated for the neutral X ‘A, ground state using standard bond lengths and angles. These are gathered in table 5 and will serve as a basis for the following discussion. As both ground state and excited state are of 2E symmetry and Jahn-Teller effects have been found to be fairly small, the Hund’s case (a) formalism used by the abovementioned authors seems appropriate. Ignoring smaII matrix elements which are responsible for L-uncoupling, X-type doubling, Idoubling and any rotational dependence of a, analytical eigenvalues have been obtained as l)+((A,-B,)P(P+

E==B,J(J+

-~&(2~+ &fB,C(Yd)‘-

1) +A”($” 2(2P+

1) + l/4) l)Yd+

(2J+

l)‘]t’2,

(11) where Y = a&,/B,, and I,, is the expectation value of the total internal vibronic angular momentum, which depends on the vibronic state, as outlined above. P and Z are good quantum numbers in the case (a) limit, for which the condition is: (Yd)‘x-

-2(2P+

l)Yd+(2J+

1)2.

(12)

Table 6 Observed J-dependent and K-dependent rotational temperatures as a function of helium stagnation pressure (nozzle diameter 70 pm) a,

P

A P/R

b’

L,(J)

4n.a. =)

(b%)

(cm- ‘)

(W

0.40 0.60 0.80 1.00 1.20 1.40

2.95

26

2.15 1.85 1.65 1.60 1.40

13 It2 10 i-1 8.5 i 1 8.011

T,m (K)

(K)” 64&43 392 13 33+10 33*11 242 2

11 8 7 6 6

&2

6.1&l

22&

5

8

a) Experimental stagnation pressure.

b’ Separation between P and R branch maxima, averaged over both spin-orbit components of both the 7: band and the 13: ‘*a*’ band. =’ Calculated from A P/R. d’ For the 0: band (see text).

(parallel band) and A J = 0 for K= 0. The rotational structure is essentially similar to that of a linear *II--‘IT transition_ The two spin-orbit components have slightly different effective B values. The clearest instance of J-dependent substructure is observed in the 13: “a” bands (fig. 4~). The spin-orbit components are separated by = 20 cm- ‘, and the P and R branches of the I\‘= 0 subband clearly stand out (the Q branch is forbidden)_ The red-shading of the rotational structure indicates an increase of IBaC and hence of 11~ overall molecular length in the excited staIe. The P and R branches of both spin-orbit components of the K = 0 subband can also be observed on the 7:

This condition

is well-fulfilled for the low rotational levels populated in this experiment (KG 3, J f 15) and for not too low values of the quenching parameter [dj > 0.25. Hence the square root in eq. (11) can be expanded: Ei=

B,(l

&-B,/aS,d)J(J

+ (A,--B”)P(P+

+ 1) 1)

---A,!&,(1 * B”/2A,5,,)(2P *B:/4aS,d+A,(l:V

+ 1) &&;d/2

+ l/4).

(13)

Thus the two spin components of the electronic state are separated by approximately aI,d, each component being associated with slightly differing effective values of B, and A,<,,.. Typically. the effective values differ by a fraction of a percent to a few percent from the average values, for different vibronic states. As K remains a good quantum number under any degree of electron spin coupling to the top axis, eq. (13) may be rewritten in terms of K by putting K = P - l/2 E+=B,(l -

-t_B,/a{,d)(J+

-2A,S,,.(l + aled/

l/$+(A,-B,)K’

f B,/2A,S,,.)K

+ A&,. .

(14)

We first discuss the J-dependent

the selection

rules are A J = 0,

substructure: f 1 for AK = 0

4

20350

I

I

20370

I

1

203eO

I

t

2039Oicm-‘1

Fig. 5. Higher-rssolution recording of the 0: band of CH,-CZC-C=C-H(optical bandpass 0.009 nm fwhm). showing the development of the rotational envelope structure with He backing pressure increasing irom 0.4 to 1.4 bar. The R-branch bandhcads of the K, k subbands bslonging IO the X = i l/2. - l/2 spin-orbit componenrs are indicated ior the scan at 0.4 bar He backing pressure. The spin-orbit splitting As0 is also indicated. _L\narbitrary sign of K is given.

the stagnation-pressure range employed to T,,,,(J) =6Kat 1.4bar. The K-dependent subband structure can be derived using eq. (14). For a parallel band (AK= 0). and neglecting J-dependent terms, one obtains for the transition energy z,(i\‘. -‘) = z’a+ [(AL. - A::..) - ( B,. - Bt::.)] K2 +2K

[ A;:..{;v - A:.{:., + ;( B,::. - B,I.)]

-+{$.A;..

- y~,A::.._

(15)

where zjr, is the purely vibronic

The constant

transition energy. terms can be included in vO, giving

for the 0: band: z*(K.Z)=P;+

[(A;-A;)-(B;-B;;)]K’ +2K[Ab’S;;--AbZ’+f(B~-B;)]_ (16)

I

I

z0370

I

I

203eo

I

The frequency difference between the Kth and the (K + l)th subband is thus: t

20390 [cm-‘]

Fig. 6. Higher-resolution recording of the 0: band of CD3 -C=C-C=C-H +. showing the der*elopment of rhe rotational envelope structure uith He backing pressure from 0.4 to 1.0 bar. (See also fig. 5).

vibronic bands. with similar P/R

branch maxima separations. The P/R branch maxima separations decrease with increasing stagnation pressure, from rypically = 3.0 cm-’ at pHe =0.4 bar to = 1.4 cm-’ to a at PHe= 1.40 bar. This corresponds decrease in rotational temperature T,,,(J). Using BN = B”,3 = I?: = 0.0681 cm-’ (from table 5) and a relative decrease of the rotational constant d B/B: = ( BA - Bg)/Bi = -0.05 one can calculate a r,,,(J) for each pressure value. These temperatures (averages of 7,” and 13: bands) are given in table 6. The rotational temperatures are actually quire insensitive to the value of 4B assumed;

variation of AB from -2.5% to -7.B changes T,,,(J) by r 10%. A B for the corresponding transition in diacetylene cation is -4.9% 1161. These variations and also the calculated Jmax values are included in table 6. Thus the rotational temperature is estimated to drop by a factor of = 4 over

+(2K+

I)[(&-A;)-(B;-I?;)].

(17)

Spectra of the vibronic origin bands at higher resolution are shown in fig. 5 for h,-PD’ and in fig. 6 for d,-PD+. The spectra were recorded at a number of different stagnation pressures ranging from pHe = 0.4-1.4 bar. The fine structure of the band is readily discernible. The characteristic spacings are 1.7- 1.8 cm-’ in the case of Ir,PD+, and O-8-0.9 cm-’ for d,-PD+. i.e. roughly half as large. in the same ratio as the respective rotational Ab/AG constants. This and the stagnation-pressure (cooling) dependence of the subband intensities establishes the presence of K-type structure.

The spin-orbit

splitting of the vibronic origins

is small, and the two spin-orbit components overlap extensively. The ASo is nearly identical for all isotopic species and does not vary with rotational temperature: thus it should be identifiable as a common feature in all the spectra of figs. 5 and 6.

This is indicated as the spacing between the K = 0 R branches, yielding Aso = 1.8 cm- ‘. The K-dependent subband structure is most evident at the lowest stagnation pressure p He = 0.40

bar. The K stacks are populated

up to IKI = 6 for

A,-PD+

and up to IKI = S for d,-PD’. as indicated in figs. 5 and 6. The subband structure on the low-energy side of the h,-PD’ origin and on the high-energy side of the d,-PD’ origin is clear at low pHe (high rotational temperature) but “washes out” at higher pressures (lower rotational temperatures). This is due to the contmclion of the P/R branch splitting of rhe different subb~nds with lower temperature. as discussed above for the example of the 13: a band with I\’ = 0. Thus some “subbands” in figs. 5 and 6 are actually coincidental superpositions of the P and R branches of neighbouring subbands. “Constructive“ superposition is observed whenever d:-+’ is equal to the P/R branch maxima separations. The Q-branch intensities are weak: the Hbnl-London factor for a parallel band Q(A J = 0) transition is given by AK,(Q) = K’/J( J -i I). which is small for the low 1K 1values and relatively high J n,il~values invoh*ed in these transitions. From the energy separations between the neighbouring subbands we can obtain by the use of eq. (17) for h,-PDT: IA;‘{;;

- /IhI; i- X( B;

- &)I

= IA;{;;

- A;{;1

= OX60 &- 0.03 cm-’

= 0.074

+ 0.003

= Iii; - II;;1

cm-‘.

In both cases the B-dependent terms are much smaller than the A-dependent terms to within the experimental error. Under the reasonable assumption that the cationic Al and AZ values are close IO the estimated A value (table 5). \\‘e further obtain an approximate value for: dS;O,,.= 111’ - {;I = 0.164

vibrationless states [eq (4)) and must arise from an admixture of exrircd sibronic 92fes 3hrough 1he Jahn-Teller effect. This supports the conclusions from the vibrational analysis rhat vibronic coupling is important in pentadiyne cations. and that the magnitude of the coupling differs bet\veen the electronic states. From the O[,’band of dl-PD6). we obtain analogouslv:

=

0.445

= !j;

- ;;! = 0.169

[no

The

nuclear-spin

mod

3 = 0 and

(b) By contrast. the total vibronic angular momenta of the vibrationless states differ consid-

Coriolis

coupling

present

in

rhs

vibrarionless ground state). Xeglecring csnrrifugal strerching terms and using the _A and B values of rahls 5. one has

very little for the A c, 2 transition,

AZ.

2 0.007

in exceilenr agreement \\-ith rhe I{:“, value of II,PD-. The rorational distribution over the different K subbands allows a second. independent_ determination of rorational temperature. The rotational distribution can be determined \vith fair accuracy for h,-PD[fig. 6). The rotarional term energies used for the temperature derermination are those of the neutral precursor molecule h,-PD_ which follow rhe simple prolate symmetric-rop

The signs of all the above quantities. which depend on the sign of K. cannot be determined directly from the spectra. The magnitudes observed show that: (a) The geometry of the methyl group chanses as d--i, = 0.01

c111- 1

again neglecting the ver\: small B-dependent terms. as for h,-PD-. Approsimaring again the cationic rotational constants as _-II; = _-lb = ;I = 3.63 cmei (table 5). we obtain for d,-PD-:

equation

& 0.006.

* 0.02

(fig.

and

_I[;,

and I( A;, - A;;) - (B;I - Bl)I

erably. According to the results of the vibrational analysis. the electronic angular momentum contributions to the total vibronic angular momentum {,V are equal for both states. as ci’ = j; = 0.64. Thus the obser\ved I{:, must be ascribed to a difference in LGbrational angular momenta in the

<;‘(J_K)=O.O6SJ(J+

rotational pressures the

1)+5_1i2K’[cm-‘1. srarisrical

xveighrs

1 for K mod

are

3 = 0. The

2 for

h’

K-_rvpe

temperatures at different stagnation are included in table 6 as T,,,( K ). For

deuteromethyl

derivatives

the

K structure

is

S. Leurwyler e: al. / Emission specrra

166

too congested to derive a reliable rotational temperature. It is notable that T,,,(K) = 2X,,,(J) for all values of pHc measured (see table 6). To our knowledge, this is the first significant evidence for IWOdifferent rotational temperatures within the same molecule cooled in a supersonic expansion. This effect is perhaps not too surprising in a molecule with such an extreme ratio of rotational constants, as A/B = 80 for ha-PD. The effectiveness of rotational cooling by R-T transfer in a supersonic jet also depends sensitively on the rotational constants of the given molecule. A large body of independent experimental evidence for the dependence of the terminal rotational temperature on the molecular rotational constants has been accumulated through recent progress in supersonic-jet spectroscopy [20]. A final point concerns the polarization of the electronic transition. Under C,, symmetry, an E c, E electric dipole transition has components both along the symmetric-top axis z and also perpendicular to it (x, y-polarization). The former component gives rise to the parallel bands (AK= 0) discussed previously; the latter component results in perpendicular bands with the selection rule AK = + 1. In addition, the vibronic selection rules as derived by Mills 1211 have to be considered for the (te), (-e) levels: A]K]= +l for E(+l)w E( - I ), giving rise to “ r” subbands, and A 1K I= - 1 for E( - I) f, E( + I), giving rise to “p” subbands. The spacings between the bands, as derived from eq. (14). are

A:+ ’ = 2[ A’( 1 + l:,) +(2K+

+ A”l;V - B’ -t Z( B’ + B”)]

l)[(A’-BB’)-(P-B”)],

(18)

on the the perpendicular band structure should exhibit three characteristic properties:

ranging from

= 2A to = 6A depending

magnitude and signs of lzy and 5::.

Thus,

(1) wider spacings than parallel band structure, (2) spacing twice as wide for h4-PD+ and d,-

PD+ as compared to d,-PD’ and d,PD’, (3) a pressure dependence similar to that of the parallel subbands discussed above.

Such structure should be especially clearly observable near the Oz, 6: and 3: bands. From the

of me~hyfdiacer,~fene carions

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