Vibrational distribution of Br2 molecule following photodissociation of
CH2Br2 by using cavity ring-down absorption spectorscopy
Pei-Ying Wei, Yuan-Pin Chang, and King-Chuen Lin*
Department of Chemistry, National Taiwan University, and Institute of Atomic and Molecular Sciences, Taipei, 106, TAIWAN
1. Experimental Setup:
Abstrate:
Cavity ring-down Spectroscopy (CRDS) is a relatively new direct absorption technique and its
applications are developed very quickly in recently years. The method is based on measurement of the
decay rate of a pulse light trapped in an optical cavity which is formed by a pair of highly
reflective(R>99.9%) mirrors. A plot of decay rate as a function of laser frequency gives the
absorptionspectrum. As for photodissociation studies of CH2Br2, the major dissociation channels are
found to be
CH2Br2 → CH2Br + Br
ΔH=60.3 kcal/mol
CH2Br2 → CH2 + Br2
ΔH=106.8 kcal/mol
We used a cavity ring-down spectroscopy (CRDS) to study of nascent Br2 following photodissociation of
CH2Br2. The quantum yield of Br2 is found to be 0.21151±0.05 following photodissociation of CH2Br2 at
248nm. According to the absorption spectrum, the nascent vibrational distribution was obtained. The
excited parent molecules (CH2Br2) may transfer into highly vibrational levels of their electronic ground
state via internal conversion. And the results agree with a given theoretical calculation.
Fig. 1 Schematic diagram of pure Br2 detection by using CRDS.
B1
2. Results and Discussions:
5
1
1 5
3 0
3 0
1 0
5
1 0
1 0
5
1 5
5
3 0
1 0
8 1 ,8 1
4 0
3 5
R
P
R
P
2 5
1
5
R
P
2 5
7 9 ,7 9
8 1 ,8 1
7 9 ,7 9
8 1 ,8 1
2 9 2 4
7 9 ,7 9
2 8
1 0
1
2
P
2 0
2 0
B r
2
R
3 5
2 5
1 5
B r
R
P
3 5
2 0
1 5
2 0
1 5
8 1 ,8 1
4 5
7 9 ,7 9
4 0
4 0
3 0
3 0
1 0
5
1
1 0
4 0
3 0
2 5
1
2 5
5
1
2 5
2 0
2 0
1
1/τ(*10-6)
3 5
3 5
2 5
1 5
2 0
3 5
2 0
3 0
1 5
1 0
1 5
4 5
R
P
1 5
1 0
5
7 9 ,7 9
1 4
B r
2
B r2
B r2
B r2
B r2
B r2
B r
2
4 .5
1250000
( 3 8 ,0 )
1200000
X
CH2+Br2
( 3 7 ,0 )
1150000
( 3 6 ,0 )
(3 5 ,0 )
1100000
1050000
1000000
(c )
3 .5
( 3 9 ,0 )
Vibrational
levels
1
1 4
5 .5
R
P
-1
2 5
2 5
2 5
6 .5
P
4 0
3 5
4 0
3 0
3 5
R
P
R
4 0
3 5
3 5
2 5
3 2
Br2intensity (s )
3 0
2 1
2 0
4 0
3 5
3 0
7 .5
950000
2 .5
295
300
(b )
305
310
315
320
325
(a)
(a )
0 .5
5 1 6 .5
5 1 7 .0
5 1 7 .5
Fig. 7 The temperature dependence
plot of photodissociation beam
intensity vs. absorption signal for
P(37) of the (35,0) band at 519.68nm.
5 1 8 .0
w a v e le n g th (n m )
Fig. 2 The cavity ring-down absorption spectrum in (v’Å0) transition of (a) Br2 without the
photodissociation beam, 248nm, at a precursor pressure of 5.1 torr, (b) Br2 with the photodissociation
beam, 248nm, at a precursor pressure of 5.1 torr, and (c) pure Br2 at pressure of 72 mtorr.
-1
1/τ(s )
R
P
8 .0 x 1 0
6
7 .5 x 1 0
6
5 27
7 .0 x 1 0
6
26
7
6 .5 x 1 0
6
6 .0 x 1 0
6
5 .5 x 1 0
6
5 .0 x 1 0
6
4 .5 x 1 0
6
4 .0 x 1 0
6
3 .5 x 1 0
6
3 .0 x 1 0
6
2 .5 x 1 0
6
2 .0 x 1 0
6
1 .5 x 1 0
6
1
27
1
1
15
10
25
1
5
10
20
20
1 2 05
1
1
5
15
10 15
5 2 2 .6
5 2 2 .8
20
20
5 2 3 .0
5 2 3 .2
5
5 2 3 .4
15
8 1 ,8 1
B r2
7 9 ,7 9
B r2
20
5 2 3 .8
( 4 1 ,1 )
B r2
8 1 ,8 1
20
19
20
7 9 ,7 9
B r2
B r2
( 4 0 ,1 )
2 87 9 , 7 9 B r
2
29
28
15
15
5 2 3 .6
8 1 ,8 1
3 43 3
25
25
10
1 01 5
(4 3 ,1 )
B r2
B r2
(4 2 ,1 )
B r2
7 9 ,7 9
(c)
(3 1 ,0 )
8 1 ,8 1
3 73 8
(b)
Fig. 8 The schematic diagram of energy
internal conversion between excited
electronic state and ground electronic
state. (a) in molecular beam, (b) valid
energy spread in bulk phase, (c) IR
multiphoton excitation to high vibration
level.
B r 2 (4 4 ,1 )
B r2
B r2
7 9 ,7 9
37 34
3 32 8
25
25
1
5
1
5
10
10
5
1
5 2 2 .4
30
30
20
20
1
5 2 2 .2
35
30
25
15
15
10
40 38
35
35
30
25
25
10
1 1 05
5
40
35
30
3 02 5
B r2
B r2
7 9 ,7 9
8 1 ,8 1
40
35
30
30
20
8 1 ,8 1
7 9 ,7 9
8 1 ,8 1
40
35
35
30
15
38
40
35
25
25
15
15
10
5
35
25
15
3 44 0
35
40
4 04 0
30
25
20
20 10
51 0
35
30
35
30
30
30
25 20
5
30
25
30
3535
25
20
1
25
20
2 53 5
30
30
25
20
15
25
20
30
23
22
23
22
15
15
17
16
10
5
20
1 53 0
10
27
5
1
15
10
X
Temperature(K)
1 .5
(3 9 ,1 )
5 2 4 .0
w a v e le n g t h ( n m )
Fig. 3 The cavity ring-down absorption spectrum in (v’Å1) transition of Br2 with the
photodissociation beam, 248nm, at a precursor pressure of 5.1 torr.
P( J ) ∝
ΣP ( J )(v" = 1)
= 0.7 ± 0.2
ΣP ( J )(v" = 0)
I (J )
( FCF )( HLF )
5.50
5.45
5.40
Slope=1.03328
R=0.99841
3
[Br2](molecular/cm )
5.35
log(I)
5.30
5.25
5.20
5.15
5.10
5.05
5.00
1.20
1.22
1.24
1.26
1.28
1.30
1.32
1.34
1.36
1.38
6.00E+015
5.80E+015
5.60E+015
5.40E+015
5.20E+015
5.00E+015
4.80E+015
4.60E+015
4.40E+015
4.20E+015
4.00E+015
3.80E+015
3.60E+015
3.40E+015
3.20E+015
3.00E+015
2.80E+015
2.60E+015
2.40E+015
2.20E+015
2.00E+015
1.40
slope=0.21151
R=0.99514
1.60E+016
log(E)
1.80E+016
2.00E+016
2.20E+016
2.40E+016
Fig. 5 Plot of number density of Br2 produced in
248 nm photolysis of CH2Br2 against the
absorbed photon density, yielding a slope
indicative of quantum yield 0.21151±0.05 for the
dissociation channel of molecular elimination.
l2
Overlapping region
Φ Br2 =
Cylindrical lens
l
[ Br2 ] =
h
Window
Fig. 10 The B3LYP/6-311G(d,p) optimized geometries of species along the
Br2 and Br dissociation channels of CH2Br2 on the adiabatic singlet ground
state potential energy surface of CH2Br2, In which the point group is in
parenthesis, the lengths in angstrom, and the angles in degree.
2.60E+016
3
[photon]248(photon/cm )
Fig. 4 The power dependence plot of
photodissociation beam intensity vs.
absorption signal for P(37) of the (35,0)
band at 519.68nm. The slope is 1.03328,
which means that Br2 is resulted from onephoton absorption of CH2Br2.
l1
Fig. 9 The Br2 and Br dissociation channels of CH2Br2, in which the energies in
kcal/mol relative to CH2Br2 are computed with CCSD(T)/cc-PVTZ level of theory with
B3LYP/6-311G(d,p) zero-point energy corrections at the B3LYP/6-311G(d,p) optimized
geometries. The energies in parenthesis are obtained at level of B3LYP/6-311G(d,p).
Window
[ Br2 ]
[ photon]248nm
d 1 1
[ − ]
clσ τ b τ a
[ photon]248nm =
Ein − Eout
hνΔV
Ein = E0 exp(−σ 248 nl1 )
Fig. 6 The magnified diagram of overlapping region. h
and l were determined by photolysis laser beam, while
the overlapping width is equal to l2 – l1.
Eout = E0 exp(−σ 248 nl2 )
248nm
(CH2Br2 →CH2+Br2)
8.88x107 s-1
(CH2Br2→CH2Br+Br)
2.50x1012 s-1
Table 1 At 248nm , the RRKM rate constant kBr2 computed with zero-point energy
corrected B3LYP/6-311G(d,p) energy and the variational RRKM rate constant k Br
computed with CCSD(T)/cc-PVTZ energy with B3LYP/6-311G(d,p) harmonic
frequencies.
3. Conclusions:
I.
The Br2 quantum yield following the photodissociation of CH2Br2 is
0.21151±0.05 at 248 nm.
II. The population ratio of Br2 in v”=1 to v”=0 at Br2 electronic ground state
is 0.7±0.2.
III. The present work under the room-temperature condition induces a much
wider population distribution in the excited state, thus enhancing the
coupling strength to facilitate the internal conversion.
IV. The theoretical calculations also can prove the Br2 channel that we have
seen in CRDS.