Absolute Photoionization Cross-Section of the Propargyl Radical
John D. Saveea, Satchin Soorkiab, Oliver Welza, Talitha M. Selbyc, Craig A. Taatjesa, and David
L. Osborna,1
a
b
c
Sandia National Labs, Combustion Research Facility, Livermore, CA 94551 USA
Institut des Sciences Moléculaires d’Orsay, Université Paris-Sud 11, Orsay, FRANCE
Department of Chemistry, University of Wisconsin, Washington County, West Bend, WI
53095 USA
Supporting Information
1
Corresponding Author: dlosbor@sandia.gov
1
1. Determination of Signal at the Time of Photolysis
As discussed elsewhere,1,2 the finite propagation time occurring between the initial
molecular-beam sampling of a photolytic product and its ultimate detection causes measured
time profiles to be perturbed from ‘true’ kinetic profiles in the reactor. Part of the temporal
instrument response function is induced by the velocity distribution of the sampled products
emerging from the reactor tube. Prior to detection, sampled species encounter two distinct
regions in the mass spectrometer where they can separate spatially according to their different
velocities: neutral species first drift ~2.4 cm from the sampling orifice to the region where they
interact with the ionizing radiation, and ions that are formed are then accelerated to ~10 eV
kinetic energy and traverse ~13.2 cm to the center of the orthogonal extraction region. For these
conditions, a simple model of the sampling process predicts average propagation times of 49.0
and 79.7 s for an initial 298 K Maxwell-Boltzmann distribution of methyl and propargyl
radicals, respectively. Because the time constant for the decay of the radical population is
substantially larger than the transit time, we can neglect the “blurring” of the time trace by the
velocity spread,1,2 and simply offset the measured data by the average propagation delay to
approximate the signal that would be measured inside the reactor.
Kinetic decays for methyl and propargyl from 193 nm photolysis of 1-butyne were
obtained at photoionization energies of 10.213 and 10.413 eV using a 1-butyne number density
of ~ 1.8 × 1013 cm-3. A 0.94% depletion of 1-butyne was observed upon photolysis and, aside
from methyl and propargyl, no other significant species arising from direct photolysis were
observed. Assuming unity quantum yield for the CH3 + C3H3 product channel, the number
densities for each of these species is estimated to be ~ 1.7 × 1011 cm-3 immediately following
photolysis. The methyl and propargyl decays from 1,3-butadiene photolyzed at 193 nm were
2
obtained using a number density of ~ 2.4 × 1012 cm-3. Photolysis at 193 nm depleted the 1,3butadiene precursor by 45%. Using a CH3 + C3H3 branching fraction of 0.5,3,4 we estimate the
initial number densities for methyl and propargyl produced by photolysis of 1,3-butadiene to be
~ 5.4 × 1011 cm-3. Dead-time effects in the OA-TOF detection system can cause an
underestimation of the fractional depletion associated with parent species, which were typically
run with high count rates.5 The number densities extracted for the photolytic products are thus
lower bounds to the actual values, and the underestimation of number densities extracted from 1butyne depletion is expected to be more significant than that from 1,3-butadiene because 1butyne produced a much higher count rate (by an observed factor of ~5).
References
(1)
S. B. Moore and R. W. Carr. Int. J. Mass Spectrom. Ion Process. 24, 161 (1977).
(2)
C. A. Taatjes. Int. J. Chem. Kinet. 39, 565 (2007).
(3)
J. C. Robinson, S. A. Harris, W. Z. Sun, N. E. Sveum, and D. M. Neumark. J. Am. Chem.
Soc. 124, 10211 (2002).
(4)
H. Y. Lee, V. V. Kislov, S. H. Lin, A. M. Mebel, and D. M. Neumark. Chem.-Eur. J. 9,
726 (2003).
(5)
P. B. Coates. Rev. Sci. Instrum. 63, 2084 (1992).
3
Table S1. Absolute photoionization cross-section spectrum obtained for the methyl radical (m/z
= 15) in the present work by scaling a relative measurement to absolute values as described in
section 3.2 of the main text.
Photoionization
Energy (eV)
9.665
9.690
9.715
9.740
9.765
9.790
9.815
9.840
9.865
9.890
9.915
9.940
9.965
9.990
10.015
10.040
10.065
10.090
10.115
10.140
10.165
10.190
10.215
10.240
10.265
10.290
10.315
10.340
10.365
10.390
10.415
10.440
10.465
10.490
10.515
ion
σmethyl
(Mb)
0.01
0.03
0.00
0.05
0.13
0.21
0.68
3.51
3.80
4.54
4.31
4.20
4.80
4.79
4.76
4.66
4.79
4.74
4.98
4.93
5.36
5.41
5.43
5.46
5.84
6.22
5.76
5.61
6.10
5.92
6.11
5.87
6.00
5.71
5.95
4
Table S2. Absolute photoionization cross-section of propargyl (m/z = 39) obtained in the present
work. A relative spectrum was scaled to absolute measurements as outlined in the text.
Photoionization
Energy (eV)
8.421
8.446
8.471
8.496
8.521
8.546
8.571
8.596
8.621
8.646
8.671
8.696
8.721
8.746
8.771
8.796
8.821
8.846
8.871
8.896
8.921
8.946
8.971
8.996
9.021
9.046
9.071
9.096
9.121
9.146
9.171
9.196
9.221
9.246
9.271
ion
σpropargyl
(Mb)
0.14
0.38
0.14
0.32
0.15
0.27
0.22
0.22
0.28
0.38
1.02
5.32
15.10
14.38
12.15
11.30
10.99
11.32
10.73
8.89
8.87
11.74
13.28
14.31
15.20
16.75
17.71
20.97
21.91
24.31
28.76
29.22
26.17
25.12
35.49
ion
σpropargyl
(Mb)
30.97
25.89
21.77
20.91
23.35
34.91
39.40
34.21
27.43
22.01
19.03
19.71
17.31
19.53
26.35
25.04
17.29
20.70
16.80
15.49
14.72
15.77
21.95
21.14
21.31
15.25
15.74
20.51
21.25
20.28
18.41
22.73
23.31
23.52
23.09
Photoionization
Energy (eV)
9.296
9.321
9.346
9.371
9.396
9.421
9.446
9.471
9.496
9.521
9.546
9.571
9.596
9.621
9.646
9.671
9.696
9.721
9.746
9.771
9.796
9.821
9.846
9.871
9.896
9.921
9.946
9.971
9.996
10.021
10.046
10.071
10.096
10.121
10.146
5
Photoionization
Energy (eV)
10.171
10.196
10.221
10.246
10.271
10.296
10.321
10.346
10.371
10.396
10.421
10.446
10.471
10.496
10.521
10.546
ion
σpropargyl
(Mb)
22.49
26.59
23.82
24.02
23.29
20.79
22.96
22.94
24.88
24.17
24.12
21.41
21.73
26.21
26.73
26.97