An angle resolved dissociative photoionization study of the c4? state

1


An angle resolved dissociative photoionization
study of the c4? state in O using the TPEPICO
technique A Padmanabhan1, M A MacDonald2, C H
Ryan2, L Zuin2 and T J Reddish1 1 Physics
Department, University of Windsor, Windsor, ON,
Canada 2 Canadian Light Source, University of
Saskatchewan, Saskatoon, SK, Canada Email
reddish_at_uwindsor.ca Web-Site
http//www.uwindsor.ca/reddish
Experimental Outline
The vibrational levels ? 0, 1 produced from
dissociative photoionization of O c4? state at
24.56 eV have distinctly different lifetimes,
??, which diminish their inherent anisotropic
photoion angular distribution characterized by a
? parameter. The ? 1 level decays to the L2
limit by tunneling through the potential barrier.
Pulse-field ionization photoelectron (PFI-PE)
experiments 1 determined the lifetime for ? 1
as 6.9 0.7 x10-14 s and this has been recently
supported by theoretical studies 2,3. In
contrast, the ? 0 level lives long enough to
fluoresce to the b4? state 4,5 and
dissociative ionization competes with radiative
decay. Akahori et al 6 also find a weak L5
contribution (5) after subtracting L5 yield due
to the underlying underlying continuum, a
background contribution that is also observed by
5,7,8. Richard-Viard et al 5 also quantify
the O/O2 ratio as 6 1 for the ? 0 level
i.e. a 15 fluorescence branching ratio.
Introduction
Angular Distributions
In the Fig 5, the angular distribution ratios are
proportional to for (a) and

for (b) the factor 2.1 comes from the (? 0
/ ? 1) threshold yield.
Fig 1 Potential Energy Curve 9 showing the
dissociation limits for O2
Vibrational Level Dissociation Products Limits Dissociation Energy (eV)
? 0 O 3P O 4S (spin-orbit coupling) L1 18.733
? 0 O 1D O 4S (tunneling) L2 20.700
? 0 O 3P O 2P (continuum) L5 23.750
? 1 O 1D O 4S (tunneling) L2 20.700
These ratios can be found by assuming that the
natural asymmetry parameter ,for a
non-rotating molecule is related to the measured
value via
with , where ? is the
rotational velocity of the molecular state and t
is its lifetime 25. The dashed blue curve is
the predicted ratio using 1.6, ?0 1.2
x10-11 s and ?1 6.0 x10-14 s , arbitrarily
normalized to the measured data. Using these ?
values, the solid red curve is fitted to the
measured data resulting in a lower value for
0.40 ? 0.05 . The corresponding values
are 0.10 ? 0.02 and 0.30 ? 0.04 for ? 0 and
? 1, respectively, and are in good agreement
with ? 0 and 0.35 observed in 25.
1 Evans et al 1998 J. Chem. Phys. 109 1285 2
Hikosaka et al 2003 J. Phys. B . 36 4311 3
Demekhin et al 2007 Rus. J. Phys. Chem. B. 2
213 4 LeBlanc 1963 J. Chem. Phys. 38 487 5
Richard-Viard et al 1987 J. Phys. B . 20 2247 6
Akahori et al 1985 J. Phys. B . 18 2219 7
Frasinski et al 1985 J. Phys. B . 18 L129 8
Ellis et al 1994 J. Phys. B. 27 3415 9 Lin
and Lucchese 2002 J. Chem. Phys. 116 8863
Fig 5(a) true coincidences corresponds
explicitly to the angular distribution ratio of ?
1 to ? 0 TPEPICO yield 5(b) random
coincidences (i.e. completely uncorrelated in
time) at h? 24.756 and 24.564 eV. The measured
black data points between 180 and 270 have been
reflected in the x and y axes to give the grey
points.
(a) (b)
Threshold photoelectron spectrum for O2
Fig 4(b) A zoomed region of the c4? state,
showing the ? 0 , 1 levels at 24.564 and 24.756
eV, respectively. As in other photoelectron
studies 8,13,14, we also find a very weak broad
feature corresponding to ? 2 at ? 24.97 eV
on the sloping background of the 2? continuum.
The energy resolution is 3.5 meV (FWHM) using He
(n 1).
Fig 4(a) The threshold photoelectron spectrum
(TPES) for O2 between 20-25 eV. The dissociative
ionization limits are indicated, as are the two
most intense vibrational series B2?   and c4? .
13 Guyon and Nenner 1980 App. Opt. 19 4068-79,
14 Baltzer et al 1992 Phys Rev A 45 4374
Results and Analysis

• Summary
• Lower limit on ?0 1 x10-12 s,
  corresponding to a width of lt 1 meV.
• ?1 6.0 0.3 x10-14 s in agreement with
  previous measurements.
• 0.40 0.05, is significantly
  smaller than predicted 9, namely
  1.6, but in good agreement with observations by
  Lafosse et al 25. values are
  0.10 0.02 and 0.30 0.04 for ? 0 and ? 1
  respectively in good agreement with 0
  and 0.35 observed in 25 using electron-ion
  vector correlation techniques.
• Our estimate of the energy width of 120
  20 meV for the ? 2 level, corresponding to ?2
  5.5 x10-15, is in excellent agreement with the
  results of recent calculations 22,23,24.

Results of energy width analysis from Fig 4(b)
and angular distribution fitting from Fig 5 in
comparison with results of previous studies.

(eV) 24.564 24.564 24.756 24.756 25.005 25.005
Theory / Exp (meV) (s) (meV) (s) (meV) (s)
21 T 6.6 x10-5 10 x10-9 0.013 5 x10-11 1.6 4 x10-13
21 T (SDCI)b 0.019 3.5 x10-11 3.6 1.8 x10-13
1 E 2.4 2.7(3) x10-13 9.5 6.9(7) x10-14
22 T 0.19 3.4 x10-12 10.4 6.3 x10-14 167 3.9 x10-15
2 Ec lt 1.1 gt 6 x10-13 9.5 6.9 x10-14
2 Tc 0.05 1.3 x10-11 9.5 6.9 x10-14
23 T 0.056 1.17 x10-11 13.2 4.99 x10-14 112 5.88 x10-15
24 T 0.054 1.22 x10-11 9.7 6.8 x10-14 142 4.6 x10-15
This Work lt ?1 gt ?1 x10-12 11.0 0.5 6.0 0.3 x10-14 120 20 ?5.5 1.0 x10-15
Funding Agencies
21 Tanaka and Yoshimine 1979 J. Chem. Phys. 70
1626 22 Liebel et al 2002 J. Phys. B. 35 895
23 Ehresmann et al 2004 J. Phys. B. 37 4405
24 Demekhin et al 2007 Rus. J. Phys. Chem. B.
2 213 25 Lafosse et al 2002 J. Chem. Phys. 117
8368-84
bSingle and double excitation configuration
interaction (SDCI). C 1.1 meV is their upper
limit from experimental observation,
corresponding to a lower limit on t0 0.05 is
an estimate from the model presented in 2 .