Dissociation dynamics of fluorinated alkyl iodides at 222 and 236 nm

Article abstract of DOI:10.1016/S0009-2614(01)00276-7

I^*(²P_1/2) quantum yield has been measured for a series of fluorinated alkyl iodides at two different wavelengths in the blue wing of their A-band. The yields are much higher for the fluorinated iodides than their corresponding alkyl iodide analogs at 222 as well as 236 nm. The I^* quantum yield at 222 nm is, in general, larger than that at 236 nm which is opposite to what has been found in normal alkyl iodide dissociation in the A-band. This difference in fluorinated iodides has been explained by invoking the participation of the B-band states in the dissociation dynamics.

Full text of DOI:10.1016/S0009-2614(01)00276-7

20 April 2001
Chemical Physics Letters 338 :2001) 118±122
www.elsevier.nl/locate/cplett
Dissociation dynamics of ¯uorinated alkyl iodides at
222 and 236 nm
K. Kavita, Puspendu K. Das *
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received 11 January 2001; in ®nal form 20 February 2001
Abstract
Ã
2
I ꢀ P1=2† quantum yield has been measured for a series of ¯uorinated alkyl iodides at two di€erent wavelengths in the
blue wing of their A-band. The yields are much higher for the ¯uorinated iodides than their corresponding alkyl iodide
analogs at 222 as well as 236 nm. The IÃ quantum yield at 222 nm is, in general, larger than that at 236 nm which is
opposite to what has been found in normal alkyl iodide dissociation in the A-band. This di€erence in ¯uorinated iodides
has been explained by invoking the participation of the B-band states in the dissociation dynamics. Ó 2001 Elsevier
Science B.V. All rights reserved.
3
1
1. Introduction
3Q1, Q0 and Q1 states in increasing order of en-
ergy. Mulliken conjectured that, of thesÃe three
3
The discovery of a photodissociation laser from
the UV photolysis of CH3I and CF3I by Kasper
and Pimental [1] in 1964 has instigated scientists to
study the photodissociation dynamics of alkyl and
¯uorinated alkyl iodides in their ®rst UV absorp-
tion band :i.e., the A-band, 210±330 nm). The
A-band of alkyl iodides arises from the transition
of a non-bonding electron of the iodine atom to a
rà molecular orbital localized on the C±I bond.
Excitation of these iodides in their A-band results
in the rupture of the C±I bond in less than 100 fs
states, only the Q0 state is correlated to I in the
dissociation limit and transition to this state is
polarized parallel to the C±I bond. The other two
3
1
states Q1 and Q1 are polarized perpendicular to
the C±I bond and produce I. Using magnetic cir-
cular dichroism :MCD), Gedanken and co-work-
ers [4,5] resolved the contribution of these three
transitions in various alkyl iodides including CF3I.
They have shown that in CF3I, the transition to
the 3Q0 state peaks at 264.1 nm and carries 84% of
the total absorption strength. Absorption to the
3
[2] to give rise to alkyl radical and iodine atoms in
2
1Q1 and Q1 states was found to be maximum at
their ground Iꢀ P3=2
†
and spin±orbit excited
237.9 and 301.8 nm, respectively.
Ã
2
I ꢀ P1=2† states. According to Mulliken [3] the
A-band is composed of three overlapping transi-
tions from the ground state :N) to the repulsive
Over the past four decades a large number of
groups [6±25] came up with a variety of techniques
to probe the dissociation dynamics of alkyl and
per¯uoroalkyl iodides in their A-band. It has been
shown that the formation of IÃ dominates the UV
photolysis of alkyl iodides and most of the avail-
able energy is released in the form of product
* Corresponding author. Fax: +91-80-3601552.
E-mail address: pkdas@ipc.iisc.ernet.in :P.K. Das).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0009 - 2 6 1 4 : 0 1 ) 0 0 2 7 6 - 7

K. Kavita, P.K. Das / Chemical Physics Letters 338 )2001) 118±122
119
translational energy. Starting from Riley et al.'s [6]
work to the latest report of Uma et al. [19], a clear
picture on the photodissociation dynamics of alkyl
iodides in their A-band as a function of size and
structure has evolved. However, the electronic ef-
fect of substitution :¯uorine, chlorine, etc.) on al-
kyl iodide dissociation dynamics has not been
studied extensively in as much detail, although the
®rst report on CF3I photolysis in the ultraviolet
appeared as early as in 1975 [7]. This may be due
to the fact that the exact excited state potential
energy surfaces :PESs) for the halogenated alkyl
iodides are dicult to generate by detailed ab
initio calculations. The demand in terms of CPU
time is very high for inclusion of the spin±orbit
coupling and the structural complexity induced by
¯uorine Ãor other halogen substitutions. In such
cases, / , the quantum yield of IÃ production, is
the only quantity which helps us understand, albeit
qualitatively, the mechanism of photodissociation.
The value of /Ã measured as a function of exci-
tation energy re¯ects the nature of the dissociation
on the excited state PESs.
Donohue and Wiesenfeld [7] measured the IÃ
quantum yield by monitoring the IR emission
from the spin±orbit excited iodine atoms pro-
duced by a broadband ¯ash photolysis of a vari-
ety of ¯uorinated alkyl iodides. Later several
techniques including infrared emission [8],
photofragment translational spectroscopy [20,21],
infrared diode±laser gain versus absorption spec-
troscopy [11], VUV laser-induced ¯uorescence [9],
resonance enhanced multiphoton ionization :RE-
MPI) [22], and pulsed-®eld time-of-¯ight :TOF)
mass spectrometry [23,24] have all been used to
measure the IÃ quantum yield from ¯uorinated
alkyl iodides. Of them, VUV laser-induced ¯uo-
rescence, REMPI, and pulsed-®eld TOF mass
spectrometry are capable of monitoring both I
and IÃ simultaneously. Earlier measurements on
¯uorinated alkyl iodides revealed that the IÃ
quantum yield is close to unity in ¯uorinated al-
kyl iodides and it does not depend much on the
structure of the per¯uoroalkyl radical. However,
more recent experiments where both I and IÃ
concentrations were monitored directly, have
shown that a small but signi®cant amount of I is
produced in the photodissociation of ¯uorinated
alkyl iodides [26,27]. However, all these investi-
gations were exclusively carried out either near
the absorption maximum or at the red edge of the
A-band. The photodissociation of ¯uorinated al-
kyl iodides in the blue wing of the A-band has not
been studied till now. In this Letter, we address
whether IÃ production from per¯uoroalkyl iodides
follows the same trend as that from normal alkyl
iodides at excitations in the blue wing of the
A-band. We have speci®cally chosen photolysis
wavelengths of 222 and 236 nm.
2. Experimental
Our experimental set-up has been described in
detail elsewhere [26]. The photodissociation
chamber was evacuated continuously with a 400
di€usion pump backed by a rotary pump to a base
pressure of 1 Â 10À6 Torr. A constant sample
pressure between 40and 70mTorr was maintained
throughout each experiment. The pump and the
probe beams were aligned perpendicular to each
other at the center of the chamber. On top of the
chamber a magnesium ¯uoride ꢀMgF2† window
was placed and a solar blind :Thorn EMI, 9433)
photomultiplier tube :PMT) was mounted above
it. The dissociation wavelengths 222 and 236 nm
were obtained by mixing the dye second harmonic
:280and 304 nm, respectively, for generating 222
and 236 nm) with the fundamental :1064 nm) of
the same Nd:YAG pump laser in a KDP crystal.
The probe laser beam :304±307 nm) was obtained
from the second harmonic of another dye laser
pumped by the second harmonic from another
Nd:YAG laser. The fragments I and IÃ were de-
tected using a two-photon laser-induced ¯uores-
cence :TPLIF) scheme [9]. The VUV ¯uorescence
from I and IÃ was collected using the solar blind
PMT, ampli®ed :typically 25 times), averaged over
30shots in a gated boxcar averager :SRS 250) and
fed to a stripchart recorder. During signal acqui-
sition the space between the PMT and the window
was continuously purged with dry nitrogen gas.
All the per¯uoroalkyl iodides were purchased from
Aldrich, USA. They were decolorized using sodi-
um bisul®te and fractionally distilled prior to each
experiment.

120
K. Kavita, P.K. Das / Chemical Physics Letters 338 )2001) 118±122
3. Results and discussion
alkyl iodides and the quantum yield is more or less
the same at both the excitation wavelengths em-
ployed here. In fact, the IÃ quantum yields from
per¯uoroalkyl iodides are found to vary within a
narrow range ꢀ0:8 Æ 0:2† over the entire A-band
:Fig. 2). In the case of CH3I dissociation at 229.4
nm, Penn et al. [15] have shown that 70% of the
The absorption spectra of the puri®ed iodides
were recorded in a Hitachi U-3000 spectrometer in
the gas phase. Sample spectra of n-C4F9I, n-C6F13I
and n-C8F17I are shown in Fig. 1. The VUV
emission spectra from I and IÃ were recorded by
scanning the probe laser across the two-photon
absorption lines of I :304.7 nm) and IÃÃ :306.7 nm).
The relative quantum yield of IÃ, / ˆ ‰IÊ=ꢀ‰IŠ‡
‰Iʆ was determined directly from the TPLIF sig-
nal intensities and is listed in Table 1. The probe
alone contribution to the TPLIF signal was
maintained at 65% in all experiments.
3
total absorption is to the Q0 state. Later Uma
et al. [19] have con®rmed for a series of normal
3
alkyl iodides that the Q0 state carries most of the
oscillator strength at 222 nm. Formation of I at
this wavelength is only possible by curve crossing
1
to the Q1 state. From the reported MCD spectra
of CF3I [5] it is known that the contribution of the
It is clear from Table 1 that IÃ formation
dominates the dissociation process in ¯uorinated
1
1Q1 state is more at shorter wavelengths. The Q1
state in CF3I peaks at 237.6 nm :close to one of the
excitation wavelengths i.e., 236 nm employed
here). In that case one would expect IÃ quantum
yield from ¯uorinated iodides to decrease at
shorter excitation wavelengths since the contribu-
1
tion of the Q1 state will become signi®cant in the
Fig. 2. The plot of IÃ quantum yield, /Ã of per¯uoroalkyl
iodides at 222, 236, 266, 280and ꢁ305 nm. The data for 266,
280and ꢁ305 nm were taken from [26].
Fig. 1. The gas phase absorption spectra of n-C4F9I, n-C6F13
and n-C8F17I.
I
Table 1
The IÃ quantum yield from per¯uoroalkyl iodides at 222 and 236 nm
Ã
/
k
:nm)
CF3I
C2F5I
n-C3F7I
n-C4F9I
n-C6F13
I
n-C8F17
I
i-C3F7I
222
236
0:71 Æ 0:0
0:72 Æ 0:0
7 :74 Æ00:0
3 :78 Æ00:0
7 :87 Æ00:0
3 :76 Æ00:0
8 :68 Æ00:0
3 :83 Æ00:0
6 :78 Æ00:0
3 :69 Æ00:0
7 :67 Æ00:0
2 :83 Æ00:0
6 :68 Æ00:06
3 :64 Æ00:02

K. Kavita, P.K. Das / Chemical Physics Letters 338 )2001) 118±122
121
blue wing excitations. But this is not found in
Table 1.
On the contrary, in some of the ¯uorinated al-
kyl iodides, there is a de®nite increase in /Ã in
going to the shorter wavelength. What may pos-
sibly be the reason for not ®nding the expected
reduction in /Ã as we go from 236 to 222 nm in
per¯uoroalkyl iodides? This could, perhaps be
explained by invoking the involvement of the next
higher excited state in the dissociation. The next
shorter wavelength absorption band to the A-band
in methyl iodide appears in the region 190±205 nm
:B-band) [28±30]. This B-band originates from the
excitation of a non-bonding 5pp valence electron
to a 6s Rydberg state centered on the iodine atom
:i.e., 6sꢀE† 5ppꢀA1† transition). The excited
3
1
state con®guration of ꢀ5pp† ꢀ6S† gives rise to ®ve
excited states [28,30]. Upon excitation the three
remaining pp electrons undergo strong spin±orbit
Fig. 3. A schematic potential energy diagram representing the
relevant A- and B-band transitions in CH3I.
‡
coupling resulting in E3=2 and E1=2CH3I ion core
well into the near ultraviolet. In fact, in CH3I the
3Q0 state which peaks at 261 nm has been found to
have signi®cant absorption at 333 [18] and even at
355 nm [27] and it leads to IÃ production at these
wavelengths. Using the same analogy we argue
that there is signi®cant absorption to the :E,1) {2}
state in per¯uoroalkyl iodides at the excitation
wavelengths of 222 and 236 nm. The contribution
of the :E,1) {2} state at 222 nm is, however, higher
than that at 236 nm :at the long end of the ab-
sorption tail). Since the :E,1) {2} state predisso-
ciates through the :E,1) repulsive state, more IÃ is
produced at 222 nm than at 236 nm by this
mechanism. This acts against the decrease in IÃ
yield which is expected purely on the basis of the
A-band excitation at 222 nm. This is, perhaps, the
reason why we do not observe a decrease in the IÃ
yield from ¯uorinated alkyl iodides :what we ex-
pect in analogy with the linear alkyl iodides) as we
go down in wavelength in the blue wing of the
A-band region.
spin±orbit states with which the Rydberg electron
then couples. In C3v symmetry these states corre-
late with E, E and A1, A2, E states and group into
two sets of states referred to as B and C bands. The
B band contains two close lying E states, which are
designated as :E,1) and :E,1) {2} where fng is a
state identi®cation index. The :E,1) state is lower
in energy and correlates with IÃ at the asymptotic
limit. Single photon transition from the 1A1
ground state to the :E,1) state is normally forbid-
den but to the :E,1) {2} state is allowed. The
transition strength in the B-band is, therefore,
carried mostly by the :E,1) {2} state. Initial exci-
tation in this band takes a methyl iodide molecule
to the :E,1) {2} state which then predissociates
through the :E,1) repulsive state [30,31] and leads
to the formation of Ià :Fig. 3). The e€ect of ¯uo-
rine substitution on the peak positions of the A
and B bands has been reported. There is a
bathochromic shift of the A-band by 7±12 nm [5]
and a hypsochromic shift of the B-band by ꢁ8 nm
[32], as we go from CH3I to CF3I. The red and the
blue shifts of the A and B bands upon ¯uorination
were attributed to the lowering of the energy of the
excited rà molecular orbital and the lone pair
electrons ꢀ5pp† on the iodine atom, respectively.
Although the B-band is blue shifted in per¯u-
oroalkyl iodides it is likely to have a tail extending
Table 1 also displays that /Ã from n-C3F7I is
more than that from i-C3F7I at 222 as well as 236
nm. This is in sharp contrast to what is seen
at dissociation wavelengths P 266 nm, where /Ã
from both normal and branched per¯uoropropyl
iodides is more or less the same [26]. The absorp-
tion pro®le of both of these iodides looks similar
in the wavelength region 220±240 nm and the

122
K. Kavita, P.K. Das / Chemical Physics Letters 338 )2001) 118±122
contribution of the 3Q0 state to the A-band is
approximately 84% in both. Therefore, the higher
IÃ yield in the normal ¯uorinated alkyl iodide must
be originating from the transition to the :E,1) {2}
state at these wavelengths. However, it appears
from the results that in the case of ¯uorinated
isopropyl iodide, this higher excited state belong-
ing to the B-band is not accessible and no IÃ is
produced through the B-band excitation mecha-
nism.
[2] M. Dzvonik, S. Yang, R. Bersohn, J. Chem. Phys. 61
:1974) 4408.
[3] R.S. Mulliken, J. Chem. Phys. 8 :1940) 382.
[4] A. Gedanken, M.D. Rowe, Chem. Phys. Lett. 34 :1975) 39.
[5] A. Gedanken, Chem. Phys. Lett. 137 :1987) 462.
[6] S.J. Riley, K.R. Wilson, Faraday Discuss. Chem. Soc. 53
:1972) 132.
[7] T. Donohue, J.R. Wiesenfeld, J. Chem. Phys. 63 :1975)
3130.
[8] J.E. Smedley, S.R. Leone, J. Chem. Phys. 79 :1983) 2687.
[9] P. Brewer, P. Das, G. Ondrey, R. Bersohn, J. Chem. Phys.
79 :1983) 720.
[10] G.N.A. van Veen, T. Baller, A.E. De Vries, N.J.A. van
Veen, Chem. Phys. 87 :1984) 405.
[11] W.P. Hess, S.J. Kohler, H.K. Haugen, S.R. Leone, J.
Chem. Phys. 84 :1986) 2143.
4. Summary
[12] F.G. Godwin, P.A. Gorry, P.M. Hughes, D. Raybone,
T.M. Watkinson, J.C. Whitehead, Chem. Phys. Lett. 135
:1987) 163.
1. The IÃ yield from per¯uoroalkyl iodides varies
within a narrow range as a function of the exci-
tation wavelength.
[13] F.G. Godwin, F. Paterson, P.A. Gorry, Mol. Phys. 61
:1987) 827.
2. The dynamics of dissociation of the ¯uorinated
alkyl iodides in the blue wing region of the
A-band is qualitatively di€erent from that of
the analogous alkyl iodides. In the former, the
formation of IÃ at 222 and 236 nm has contribu-
tions from two sources: :i) the direct excitation
to the 3Q0 state lying within the A-band, and :ii)
predissociation from the :E,1) {2} state lying
within the B-band.
[14] L.R. Ogorzalek, G.E. Hall, H.P. Haerri, P.L. Houston, J.
Phys. Chem. 92 :1988) 5.
[15] S.M. Penn, K.J. Hayden, C. Muyskens, F.F. Crim, J.
Chem. Phys. 89 :1989) 2909.
[16] S. Uma, P.K. Das, Can. J. Chem. 72 :1994) 865.
[17] J.A. Syage, R.A. Hertz, J. Chem. Phys. 100 :1994) 9265.
[18] D. Howard Fairbrother, K.A. Briggman, E. Weitz, P.C.
Stair, J. Chem. Phys. 101 :1994) 3787.
[19] S. Uma, P.K. Das, Chem. Phys. Lett. 241 :1995) 335.
[20] D. Krajnovitch, L.J. Butler, Y.T. Lee, J. Chem. Phys. 81
:1984) 3031.
3. At 222 and 236 nm, the IÃ quantum yield from
i-C3F7I is less than that from n-C3F7I. The IÃ
production in n-C3F7I, perhaps, involves simul-
[21] M.D. Person, P.W. Kash, L.J. Butler, J. Chem. Phys. 94
:1991) 2557.
[22] H.J. Hwang, M.A. El-Sayed, J. Chem. Phys. 94 :1991)
4877.
3
taneous excitations to two states ꢀ Q0† and
:E,1) {2} whereas that in i-C3F7I involves exci-
tation exclusively to the Q0 state.
[23] A. Furlan, T. Gejo, J. Robert Huber, J. Phys. Chem. 100
:1996) 7956.
3
[24] W.K. Kang, K.W. Jung, D.C. Kim, K.H. Jung, J. Chem.
Phys. 104 :1996) 5815.
[25] P. Felder, Chem. Phys. 143 :1990) 141.
[26] K. Kavita, P.K. Das, J. Chem. Phys. 112 :2000) 8426.
[27] A.V. Baklanov, M. Aldener, B. Lindgren, U. Sassenberg,
Chem. Phys. Lett. 325 :2000) 399.
Acknowledgements
The work described here has been generously
funded by the Department of Atomic Energy,
Government of India.
[28] S. Felps, P. Hochmann, P. Brint, S.P. Mcglynn, J. Mol.
Spectrosc. 59 :1976) 355.
[29] J.A. Syage, J. Steadman, J. Phys. Chem. 94 :1990) 7343.
[30] G.N.A. van Veen, T. Baller, A.E. De Vries, Chem. Phys. 97
:1985) 179.
References
[31] R.E. Continetti, B.A. Balko, Y.T. Lee, J. Chem. Phys. 89
:1988) 3383.
[32] M.B. Robin, Higher Excited States of Polyatomic Mole-
cules, vol. 1, Academic Press, New York, 1974, p. 166.
[1] J.V.V. Kasper, G.C. Pimentel, Appl. Phys. Lett. 5 :1964)
231.