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
† Web-Site:
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
We have investigated the angular distributions of ~2 eV O+(4S) ions following dissociative photoionization from the c4 state for both  = 0 and 1 using the threshold photoelectron-photoion coincidence (TPEPICO) technique. Due to axial recoil in a diatomic molecule, the ion energy is simply given by: = (h - D) / 2 , where D is the dissociation limit. The threshold photoelectron yield peaks at h = and eV for  = 0 and 1 levels, respectively, the corresponding values of are and eV for the dissociation limit of 20.7 eV. The toroidal analyzer used to detect ions was operated with an energy resolution of  E = 0.5 eV, which is much broader than the ~100 meV spacing when set to detect 2.0 eV ions, and can readily separate ions from the neighbouring dissociation limits at and eV. E O + E O +
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
O 1D + O+ 4S
(tunneling) L2
20.700
O 3P + O+ 2P
(continuum) L5
23.750
 = 1
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 τ is its lifetime [25]. The dashed blue curve is the predicted ratio using = 1.6, 0 = 1.2 x 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  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
[2] Hikosaka et al 2003 J. Phys. B
[3] Demekhin et al 2007 Rus. J. Phys. Chem. B
[4] LeBlanc 1963 J. Chem. Phys
[5] Richard-Viard et al 1987 J. Phys. B
[6] Akahori et al 1985 J. Phys. B
[7] Frasinski et al 1985 J. Phys. B . 18 L129
[8] Ellis et al 1994 J. Phys. B
[9] Lin and Lucchese 2002 J. Chem. Phys
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 = and eV. The measured black data points between 180° and 270° have been reflected in the x and y axes to give the grey points.
Toroidal Spectrometer
A Threshold Photoelectron-Photoion-Coincidence (TPEPICO) experiment was performed using a dual toroidal spectrometer [10] in conjunction with linearly polarized synchrotron radiation on the VLS-PGM 11ID-2 beamline at the Canadian Light Source (CLS) [11]. The spectrometer utilizes toroidal analyzers, which have properties ideally suited for measuring charged particle angular distributions since they energy select the photoions while preserving the initial angle of emission. The threshold electrons were obtained using the penetrating field technique [12], which is explained in Fig 3, in conjunction with the smaller toroidal analyzer shown in Fig 2.
Fig 2: A schematic diagram of the mutual configuration of the two toroidal analyzers in our detection geometry. The photon beam is perpendicular to the page and the polarization direction is horizontal. The TPEPICO signal corresponds to threshold electrons yield (over 4 sr) measured in coincidence with energy-resolved ions with emission angles within the central ~160 grey sector of the (180) toroidal analyzer. The out-of-plane emission angular acceptance in the ion channel is ~ ±10°.
[10] Reddish et al Rev. Sci. Instrum
[11] Hu et al Rev. Sci. Instrum
[12] Cvejanovic and Read  1974 J. Phys. B
Fig 3: Trajectories of 0.001eV electrons emitted over 4 sr from a point source can be focused and collimated by the weak electric field from an ‘extractor’ electrode that penetrates through the zero volt aperture. The solid angle of extracted, faster electrons is significantly smaller than for these "threshold" electrons and rapidly diminishes with electron energy. This highly-efficient, energy selective extraction allows 'threshold electron spectroscopy’.
(a) (b)
[15] Sadeghpour et al 2000 J. Phys. B . 33 R93
[16] King and Avaldi 2000 J. Phys. B . 33 R215
[17] Bouri et al 2007 J. Phys. B . 40 F51
[18] Cvejanovic et al 1995 J. Phys. B . 28 L707
[19] Wehlitz et al 1999 J. Phys. B . 32 L635
[20] Thompson et al 1998 J. Phys. B
Fig 6: Threshold photoelectron spectrum (TPES) of Helium. Insert: The characteristic “cusp” [16,18] at the double ionization threshold energy of eV.
Threshold Photoelectron Spectrum of Helium
The double ionization region in helium continues to be the subject of intense interest [15-20], since it is the archetypal electron correlation system. Fig 6 shows the well-studied threshold region displaying the characteristic cusp at 79 eV, obtained with a resolution of  10 meV. The ratio of the threshold yield immediately below and above 79 eV is  1.08, in good agreement with our earlier study [18] obtained with ~ 70 meV resolution and still at variance with the  1.25 value from [20].
Threshold photoelectron spectrum for O2+
Fig 4(b): A zoomed region of the c4 state, showing the  = 0 , 1 levels at and eV, respectively. As in other photoelectron studies [8,13,14], we also find a very weak broad feature corresponding to  = 2 at  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 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 , [14] Baltzer et al 1992 Phys Rev A
Results and Analysis
Summary
Lower limit on 0 ~1 x10-12 s, corresponding to a width of < ~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
24.756
25.005
Theory / Exp
(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
< 1.1
> 6 x10-13
6.9 x10-14
[2] Tc
0.05
1.3 x10-11
[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
< 1
> 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
[22] Liebel et al 2002 J. Phys. B
[23] Ehresmann et al J. Phys. B
[24] Demekhin et al 2007 Rus. J. Phys. Chem. B
[25] Lafosse et al 2002 J. Chem. Phys
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 ; is an estimate from the model presented in [2] .