Institut für Physikalische
und Theoretische Chemie
Quantum state selective detection of molecular chlorine
by high-resolution cavity ring-down spectroscopy
Thomas Forsting, Ignacio Vespoli, Mathias Piescheck, Christof Maul
Technische Universität Braunschweig | Institut für Physikalische und Theoretische Chemie
[email protected] | Telefon +49 (0) 531 391-5345
Experimental Setup and Parameters
Introduction
State resolved detection of nascent products of chemical reactions and/
or photodissociation provides valuable information about the dynamics of
the underlying elementary processes. Due to the lack of suitable optical
detection methods molecular elimination processes of the diatomic halogens have only rarely been studied.[1, 2] For chlorine in particular no such
data exist up to date.
general scheme of our CRD-setup:
experimental parameters:
general:
- temperature: T = r. t. (approx. 293 K)
- gas pressure: 30 mbar Cl2 in
N2, He, CO2 (up to 970 mbar)
308 nm
DAQ-system
A promising candidate for state-specific detection of molecular chlorine as nascent photoproducts from an elementary chemical reaction is
the (spin-forbidden) transition B3P(0u+) f X1Sg+. Its weak, rovibrationally
resolved bands lie in the visible range with absorption cross-sections of
about 10-21 to 10-22 cm2.[3]
XeCl-eximer
dye laser
PMT
photomultiplier
CRD-chamber
MFC
200 sccm/N2
vacuum
perturber
gas
MFC
Cl2
50 sccm/N2
Cl2-Background Information
UV/Vis (0.2 nm) - and BBCEAS (0.05 nm)[3] Spectra:
25
1
C Πu
Cl+Cl
B 3Π 0 +u
2
Cl+Cl
A 3Π 1u
2
*
1
energy / 104 cm-1
energy / eV
3
1
1
+
X Σg
0
1.8
2.4
3.0
3.6
R (Cl-Cl) / angstrom
4.2
20
15
10
Cl2: 57.4 %,
35 37
Cl Cl: 36.7 %
37
Cl2 5.9 %
(natural abundance)
35
0,016
0,014
0,012
0,010
0,004
0,002
530
540
550
560
570
wavelength / nm
5
0
200
300
400
500
600
700
wavelength / nm
P(32)
2,0
1,0
0,5
0,0
510,0
510,5
19596
511,0
wavelength / nm
19584
19572
19560
wavenumber / cm
-1
511,5
512,0
P(30)
P(31)
2,0
R(34)
1,5
19608
2,5
R(32)
R(33)
P(28)
P(29)
R(31)
0,5
19550
19536
19560
19570
19580
-1
We present experimentally observed spectra with a spectral resolution of 0.0006 nm which contain lines of all the three
isotopologues 35Cl2, 35Cl37Cl and 37Cl2 according to their natural abundance.
Simulation was carried out with spectroscopic constants taken from [7] and [8] and the peak assignment of measured
data is still in progress, thus line assignment above is still tentative.
In conclusion, we present a quantum state resolved spectrum of molecular chlorine with the highest resolution known to date. Our measurements of molecular chlorine exeed the best known
spectral resolution (0.04 nm)[9] by a factor of about 60.
For the first time, perturber-gas broadening coefficients for chlorine are reported.
Our results comply with self-broadening coefficients of Cl2 in the literature.
The current detection limit of the setup is of the order of 1014 cm-3.
[1] P.-Y. Wei, Y.-P. Chang, Y.-S. Lee, W.-B. Lee, and K.-C. Lin, J. Chem. Phys. 126, 034311 (2007).
[2] S.-Y. Chen, P.-Y. Tsai, H.-C. Lin, C.-C. Wu, K.-C. Lin, B.J. Sun, and A.H.H. Chang, , J. Chem. Phys. 134, 034315 (2011).
[3] I.A.K. Young, C. Murray, C.M. Blaum, R.A. Cox, R.L. Jones, and F.D. Pope, Phys. Chem. Chem. Phys. 13, 15318 (2011).
[4] R. C. Weast (ed.), CRC Handbook of Chemistry and Physics, 61st edition, Florida: CRC Press (1980-1981).
[5] D. Zhang, A. Abdel-hafiez, B. Zhang, Chem. Phys. Lett. 428, 49 (2006).
wL = 0.109(6) cm-1
(FWHM)
0,6
19577,0
19577,2
19577,4
19577,6
19577,8
N2
He
CO2
0,16
0,12
0,08
0,04
0,00
0
200
400
600
800
1000
pressure / mbar
B3P(0u+) (v‘ = 10) f X1Sg+(v“ = 0)
Discussion and Conclusion
0,9
0,20
1,0
0,0
19540
1,2
0,24
R(28)
1,5
wavenumber / cm
19548
R(29)
R(30)
double-voigt fit
- linear fit of wL over the pressure result into perturber-gas coefficients g:
P(26)
P(27)
+ 120 mbar N2
1,5
wavenumber / cm-1
line width (FWHM) / cm-1
2,5
absorption cross-section / 10-22 cm2
absorption cross-section / 10-22 cm2
this work
Young et al. (2011)[3]
30 mbar Cl2
0,3
19576,8
High-Resolution Spectrum (Dl = 0.0006 nm)
Voigt-function:
convoluted Lorentz and Gauß-function
1,8
0,006
520
12 bit / 350 MHz
- pressure-broadening is Lorentz-distributed
- Voigt-fit circumvents Gauß-distributed signal content
(laser line-width and Doppler-broadening) by fixing wG:
0,008
0,000
510
0
isotopologues[4]:
0,018
absorption cross-section / 10-20 cm2
3
absorption cross-section / 10-20 cm2
4
data acquisition:
- resolution:
Pressure-Broadening
cavity loss (c )-1 / 10-4 cm-1
potential energy curves[5]:
laser & optics:
- pump source: XeCl (308 nm) @ 20 Hz
- dye range: approx. 485 - 546 nm
- laser band-width: 0.2cm-1
- mirror reflectivity: 0.9998
perturber-gas broadening coeffcients of chlorine:
g (Cl2, N2) = 0.165(2) cm-1/barCl2-self-broadening coefficient:[6]
g (Cl2, He) = 0.137(9) cm-1/bar g(Cl2) = 0.203(8) cm-1/bar
g (Cl2, CO2) = 0.159(4) cm-1/bar
The obtained absorption cross-sections are fully consistent to those reported in literature.[3, 9]
Altogether, we show that CRD spectroscopy is a powerful and yet very easy method to obtain high
resolution gas-phase absorption spectra of species with very low absorption cross-sections.
The results are a proof of concept and a preliminary work on the way to the measurement of the
dynamics of nascent chlorine.
[6] K. K. Innes, S. J. Jackling, III, T. W. Tolbert, J. Quant. Spectrosc. Radiat. Transfer. 16, 443 (1976).
[7] G. Herzberg, K. P. Huber: Constants of Diatomic Molecules. New York, London: Van Nostrand Reinhold (1979).
[8] J. A. Coxon, J. Mol. Spec. 82, 264 (1980).
[9] D. Maric, J. P. Burrows, R. Meller, and G. K. Moortgat, J. Photochem. Photobiol. A: Chem. 70, 205 (1993).