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Resonant-Auger-state-selected dissociation dynamics and dissociation limits of N 1s→π* core excited N2 molecules studied using a two-dimensional Auger-electron-photoion coincidence method
Journal of Electron Spectroscopy and Related Phenomena 232 (2019) 40–44
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Resonant-Auger-state-selected dissociation dynamics and dissociation limits of N 1s→π* core excited N2 molecules studied using a two-dimensional Auger-electron-photoion coincidence method
T
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Hiroshi Iwayamaa,b, , James R Harriesc a
UVSOR Facility, Institute for Molecular Science, Nishigonaka 38, Myodaiji, Okazaki, 444-8585, Japan SOKENDAI, Nishigonaka 38, Myodaiji, Okazaki 444-8585, Japan c QST, SPring-8, Kouto 1-1-1, Sayo, Hyogo, 679-5148, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photodissociation Core excitation Auger-electron-photoion coincidence
The dissociation dynamics of core-excited N2 molecules has been investigated using a two-dimensional Auger electron photoion coincidence method, where the kinetic energies of Auger electrons and ions are measured in coincidence. From the energy correlations between the binding energies of the resonant Auger states and the ion kinetic energy release, the electronic states of fragments at the dissociation limit are investigated for each Auger peak. We also estimate the contribution of double Auger decay to the total Auger spectrum by analyzing events where two N+ ions or metastable doubly charged ions N2++ were detected.
1. Introduction The understanding of dissociation pathways is one of the most important topics in the study of the photoionization of molecules [1,2]. Since photoionization can lead to the production of a range of different cationic states, the subsequent relaxation and fragmentation process is generally complex. To understand the overall photodissociation process, it is thus important to reveal the links between the initial cationic states and the corresponding final products. Electron-ion coincidence measurements [3–9] have proved to be one of the best methods to investigate this state-selected dissociation dynamics. Photoionization in the vacuum ultraviolet (VUV) wavelength region has been widely used to study the dissociation processes of outer-valence ionized molecules. The groups of Nahon [3] and Sheng [4] have developed techniques for double-imaging photoelectron-photoion coincidence measurements, where electrons and ions are detected in coincidence using two velocity map imaging spectrometers. This allows the study of state-selected dissociation dynamics and the determination of dissociation limits, and provides information on not only the masses and charge states of fragment ions but also their electronic states. These advances have breathed new life into valence photoionization measurements, for example by providing new details on the mechanism by which the free halogen atoms F and Cl, which can deplete ozone in the stratosphere, are produced by the photodissociation of CH3X, for X = F [10], and X = Cl [11].
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The inner-valence photoionization of simple diatomic molecules has also been investigated using a high-resolution threshold photoelectronphotoion coincidence technique [5,6]. Since threshold photoelectrons have nearly zero kinetic energies, the binding energy of the initial cations is almost equal to the incident photon energy. Thus tuning the photon energy corresponds to selecting the initial cationic state. These high resolution measurements were successful in identifying the dissociation limits for some inner-valence ionic states, and the state-selected dissociation dynamics were compared to configuration interaction calculations. At higher photon energies, the absorption of soft x-rays can result in core excitation and inner-shell ionization of molecules [2]. For light elements, the dominant decay process of core-excited or core-ionized molecules is Auger decay. Similiarly to VUV photoionization, the initial ionic states can be deduced from the kinetic energies of the Auger electrons, and the Auger-electron-photoion coincidence (AEPICO) technique is widely used. However, in general, the Auger electrons emitted following soft X-ray absorption have high kinetic energies, making velocity momentum imaging and threshold photoelectron techniques difficult to apply. Morin et al. developed an AEPICO experimental setup combining a double toroidal electron analyzer and an ion momentum spectrometer [12,13]. The use of a double toroidal electron analyzer provides both high energy resolution and high detection efficiency for high energy electrons such as Auger electrons. The technique was used to analyze resonant-Auger-state-selected
Corresponding author. E-mail address: [email protected] (H. Iwayama).
https://doi.org/10.1016/j.elspec.2019.01.005 Received 14 November 2018; Received in revised form 14 January 2019; Accepted 23 January 2019 Available online 24 January 2019 0368-2048/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electron Spectroscopy and Related Phenomena 232 (2019) 40–44
H. Iwayama, J.R. Harries
dissociation dynamics, and has provided insight into the understanding of the relaxation dynamics following core excitation in molecules, such as ultrafast dissociation [14] and site selective dissociation [15,16]. We have developed a similar AEPICO spectrometer and recently reported on the stability and dissociation dynamics of core ionized N2 molecules [17]. Here, we extend this work, and present experimental results on the dissociation dynamics following 1s→π* excitation of N2. The results provide extensive information on both dissociative and nondissociative resonant Auger states. The AEPICO spectrometer enables us to measure the kinetic energies of Auger electrons and ions in coincidence for each photoionization event. By analysing the kinetic energies of the Auger electrons and product ions, we obtain two dimensional AEPICO maps showing the energy correlations between the binding energies of the resonant Auger states and the kinetic energy release (KER) of the fragments. This allows us to determine the dissociation limit for each resonant Auger state. In addition, we also estimate the relative abundance of double Auger decay in the total Auger spectrum.
Fig. 2. Total Auger electron spectrum following N 1s→π* excitation, recorded using a pass energy of 400 eV. State assignments for peaks labelled A – E are given in Table 1.
with a kinetic energy between 345 eV and 390 eV. Two peaks are observed, due mainly to parent ions N2+ and fragment ions N+. Most N+ fragment ions come from dissociation pathways N2+ → N+ + N. However it is also possible for core excited N2* molecules to undergo double Auger decay, ejecting two electrons, and leading to the pathway N2++ → N+ + N+. Further, metastable doubly charged parent ions N2++ ions, which can also be produced following double Auger decay, have the same mass to charge ratio as N+ ions. The contributions of double Auger decay are discussed further below. Fig. 2 shows the corresponding total Auger electron spectrum, recorded at a pass energy of 400 eV. Kinetic energy (bottom axis) and binding energy (top axis) scales are shown. The binding energy is calculated as hv – eKE, where hv is the photon energy (401.1 eV), and eKE is the electron kinetic energy. While the peak height at the kinetic energy of 385 eV is considerably lower due to the relatively low energy resolution (∼ 4 eV), spectral features are similar to those in previously reported resonant Auger spectra [21,22], and the peaks labelled A–E can be assigned to the main configurations listed in Table 1 [23]. The valence electronic configuration of the neutral ground state of N2 is (2σ g)2(2σ u)2(1π u)4(3σ g)2, where 2σ g is termed an inner-valence orbital, and 2σ u, 1π u, and 3σ g outer-valence orbitals. Peak A, at the lowest observed binding energy of around 17 eV, is assigned to the production of the X2Σ g+(3σ g−1) and A2Π u(1π u−1) states of N2+ by participator Auger decay. The production of the third and fourth lowest electronic states of N2+, B2Π g(3σ g-21π g1) and C2Σ u+(3σ g−11π u-1 1π g1) by spectator Auger decay are the main components of peak B. In addition, peak B also includes various electronic states originating from configurations of 3σ g-21π g1, 3σ g−11π u−11π g1 and 1π u-21π g1. Peak C is mainly due to states with a main configuration of 2σ u−11π u−11π g1, and peaks D and E are attributed to states with a hole in the innervalence orbital 2σ g. The use of resonant Auger decay has some advantages in producing inner-valence excited states, which are difficult to access via the direct valence photoionization because of their weak coupling to the direct ionization channel. In Fig. 3 we show the spectra for electrons detected in coincidence
2. Experimental setup The experiments were performed on the soft x-ray beamline BL6U at UVSOR in Japan. The radiation from an undulator was monochromatized by a variable-included-angle varied-line-spacing plane grating monochromator. The photon energy was set to 401.1 eV, which corresponds to N 1s→π* core excitation [18]. The experimental setup for AEPICO measurements comprised a double toroidal electron analyzer [12,13] and an ion momentum spectrometer, each of which is equipped with a time- and position-sensitive detector (RoentDek DLD40 [19]). The full details of the design and operation of the apparatus are described elsewhere [17,20]. The electron detector covered an energy range corresponding to 15% of the pass energy. For electron kinetic energies ranging from 345 to 390 eV, we performed AEPICO measurements at a pass energy of 400 eV, which corresponds to an energy resolution of about 4 eV. For electron kinetic energies ranging from 365 to 375 eV, we also made measurements with high electron energy resolution (about 1 eV), using a pass energy of 100 eV. When an electron was detected, a pulsed extraction field of 270 V/cm was applied to the photoionization region in order to extract ions towards the ion momentum spectrometer. The extraction field was high enough to collect all ions with kinetic energies of less than 6 eV. The detection efficiency of the ion detector was estimated to be about 40%. 3. Results and discussion Fig. 1 shows a time-of-flight spectrum for ions detected following the production of core excited N2*(1s−11πg) molecules at a photon energy of 401.1 eV, and in coincidence with the detection of an electron
Table 1 Peak assignments and main configurations of cationic states in the KVV Auger spectrum of core excited N2 [23].
Fig. 1. Ion time-of-flight spectrum recorded following N 1s→π* core excitation at a photon energy of 401.1 eV. This spectrum shows the times-of-flight of ions detected in coincidence with Auger electrons with kinetic energies ranging from 345 to 390 eV. 41
Label
Main configurations of N2+ states
A B C D E
3σ g−1, 1π u−1 3σ g−21π g1, 3σ g-11π u-11π g1, 1π u−21π g1 2σ u−11π u−11π g1 2σ g−1 2σ g−11π u−11π g1, 2σ g−12σ u−11π g1, 2σ g−13σ g−11π g1
Journal of Electron Spectroscopy and Related Phenomena 232 (2019) 40–44
H. Iwayama, J.R. Harries
Fig. 4. Kinetic energy of N+ and N2++ ions above the binding energy of 42 eV.
corresponds to fragment N+ ions resulting from N2+ → N+ + N or N2++ → N+ + N+. Thus we can assume that ions corresponding to the sharp peak at 0 eV are metastable N2++ ions. The relative abundance of metastable N2++ is shown by the green filled area in Fig. 3b. The production of metastable N2++ ions is also clearly revealed in the twodimensional AEPICO maps presented below. The proportion of N+ ions resulting from N2++ → N+ + N+ dissociation can be estimated from the countrate of events where two N+ ions are detected in coincidence. To compare the event rates of electron-ion and electron-ion-ion coincidence detections, we need to take into account the ion detection efficiency and the detector dead time. The ion detection efficiency was estimated to be (40 ± 2) %, from the probability that N2+ ions were detected at a binding energy of 18 eV. To estimate the rate of false electron-ion-ion coincidence detections due to the ion detector’s dead time of 40 ns, we simulated the times-of-flight differences of two N+ fragment ions with kinetic energies of 4 ± 1 eV for 100,000 fragment pairs. The estimated false rate was (60 ± 6) %. The blue filled area in Fig. 3b represents the estimated contribution of N2++ → N+ + N+ events. The probability of double Auger decay is (19 ± 3) % for 1s→π* core excitation of N2 molecules. This value is much higher than that for N2 core ionization, which is (9.4 ± 0.9)% [26,27]. This difference may be explained by the fact that core excitation can follow spectator Auger decay, with binding energies that often exceed the double ionization threshold. To investigate the energy correlations between Auger electrons and fragment ions we present two-dimensional AEPICO maps in Fig. 5 for N2+ → N+ + N, where the vertical and horizontal axes represent the ion KER and the binding energy of the resonant Auger states, respectively. The KER plotted here corresponds to twice the kinetic energy determined for the detected N+ ions, since the two fragments have the same kinetic energy due to momentum conservation. The AEPICO maps allow us to discuss the dissociation limits of the Auger states corresponding to peaks B2, C, D and E shown in Fig. 3(b). Fig. 5(a) shows the AEPICO map covering the binding energy range from 20 to 55 eV, with the electrons detected using a pass energy of 400 eV (energy resolution ∼ 4 eV). Fig. 5(b) shows the AEPICO map corresponding to the high resolution (∼ 1 eV) data, recorded using a pass energy of 100 eV. Fig. 5(c) shows the spectrum obtained by integrating the AEPICO map of (a) in the vertical direction, which is same to the coincidence Auger spectrum for N+ ions in Fig. 3(b). The maps show several diagonal structures with a slope of 1, indicating strong correlations between Auger-electron binding energies and ion KERs. These energy correlations come from the energy conservation, KER = BE – DLE, where KER is the kinetic energy release of the fragments, BE is the binding energy of the resonant Auger states, and DLE is the dissociation limit energy. The dissociation limit energy corresponds to the binding energy at the dissociation asymptote, and depends on the electronic states of the atomic fragments as well as their charge states. The diagonal dashed lines in Fig. 5 indicate several dissociation limits. Possible dissociation limits for N2+ → N+ + N below 38 eV, obtained from Ref [6,28], are listed in Table 2. In Fig. 5(a), we also show the
Fig. 3. Coincidence Auger electron spectra for (a) parent N2+ ions and (b) fragment N+ and metastable N2++ions. The filled areas represent estimates of the contributions of N2++ → N+ + N+ (blue) and metastable N2++ ions (green) resulting from double Auger decay.
with N2+ (parent) ions (a), and fragment N+ ions (b). The energy of the lowest dissociation limit, N2+ → N+(3P) + N(4S), is also indicated. It is clear that this dissociation limit corresponds to a marked change in the coincidence Auger spectra. For binding energies below the lowest dissociation limit, only the parent ions, N2+, are detected. This can be associated with peak A, (X2Σ g+(3σ g−1) and A2Π u(1π u−1) states), and part of peak B. We label the non-dissociative and dissociative components of peak B peaks B1 and B2, respectively. The peak positions of B1 and B2 are 24 eV and 26 eV, and they can be mainly assigned to the electronic states B2Π g(3σ g-21π g1) (B1) and C2Σ u+(3σ g−11π g−11π g1) (B2). The stability of the B2Π g state (peak B1) with respect to dissociation is due to the binding energy (24.2 eV) being lower than the lowest dissociation limit energy (24.3 eV). Previous studies on outer valence photoionization [24] showed that, in addition to the B2Π g states, the C2Σ u+ state with v = 2, lying below the dissociation limit, is also stable, but that C2Σ u+with v = 3…7 dissociates into N2+ → N+ + N (states with v < 2 were not observed). While the vibrational structure of the C2Σ u+ state is not resolved in peak B, our results are consistent with the production of C2Σ u+ states mainly leading to dissociation. Above the lowest dissociation limit energy, the number of electrons detected in coincidence with parent ions N2+ rapidly decreases, with almost all of the created N2+ ions dissociating and producing N+ fragments. Below the N2 double ionization threshold of 42 eV [25], the counterpart to the N+ fragment is a neutral N atom, but above this threshold double Auger decay can also occur, leading to the production of N+ + N+ fragment pairs or metastable ions N2++. In our experiment we were unable to detect low energy electrons resulting from this double Auger decay, due to the high background signal from scattered electrons. The stray light of incident soft x-rays would hit electrodes and produce many satterted electrons. We discuss below the contributions from double Auger decay. Fig. 4 shows the kinetic energy distribution of ions with m/q = 14 amu/e− (N+ or N2++) detected above the double ionization threshold of 42 eV. A sharp peak can be seen 0 eV, and a broad peak at 4 eV. The sharp peak around 0 eV can be attributed to parent N2++ ions which are metastable and are detected before dissociation.The broad peak 42
Journal of Electron Spectroscopy and Related Phenomena 232 (2019) 40–44
H. Iwayama, J.R. Harries
lowest dissociation limit of doubly charged ions, N2++ → N+(3P) + N+(3P), which lies at 38.8 eV [17]. First we discuss the dissociation pathways of the resonant Auger states corresponding to peak B2 of Fig. 5(c). The two-dimensional AEPICO map (Fig. 5b) reveals a diagonal island-like structure centred at a binding energy of around 26 eV. This peak corresponds to the C2Σ u+(3σ g−11π u−11π g1) states. The corresponding dissociation limit is the lowest dissociation limit 1 (N2+ → N+(3P) + N(4S°)). The diagonal island-like structure extends to a KER of 0 eV, showing that dissociation begins to occur once binding energies reach this limit. A further diagonal island-like structure can be seen also beginning at a kinetic energy release of 0 eV, with a binding energy of around 27 eV. The results suggest that these dissociative states are associated with dissociation limit 3 (N2+ → N+(3P) + N(2D°)) rather than dissociation limit 2 (N2+ → N+(1D) + N(4S°)). A third diagonal island-like structure can be seen in Fig. 5b, which can be associated with peak C in Fig. 5(c). While our electron resolution (∼ 1 eV) makes it difficult to completely separate this structure from that assigned above to states related to dissociation limit 3, this structure appears to be also associated with dissociation limit 4. For binding energies above 30 eV, the main dissociation limit for the resonant Auger states (2σ u−11π u−11π g1) which make up peak C is dissociation limit 4 (N2+ → N+(3P) + N(2P°)). Shifting our focus to Fig. 5(a), which shows data recorded with a pass energy of 400 eV (electron energy resolution ∼ 4 eV), we first discuss the dissociation dynamics of states with binding energies of around 36 eV. These resonant Auger states correspond to peak D of Fig. 5(c), and are electronic states with a hole in the inner-valence orbital 2σg. Diagonal structures at KERs of around 2 eV and 8 eV can be associated with these states, and lie along the dashed lines 7 (N2+ → N+(2p−1 1D) + N(2P°)) and 11 (N2+ → N+(2 s−1 3D°) + N(4S°)). This suggests the interpretaion that upon dissociation, the inner-valence hole in the molecular 2σg orbital becomes a hole in either the atomic 2 s or 2p orbital. This is understandable since the 2σg molecular orbital has its origins in a mixing of atomic 2 s and 2p orbitals. Finally we discuss the broad diagonal and horizontal structures which can be seen for binding energies of around 47 eV, corresponding to peak E in Fig. 5(c). In this energy region, the AEPICO map includes significant contributions from double Auger decay. The horizontal structure for KERs close to 0 eV can be assigned to metastable doublycharged parent ions, N2++. The broad diagonal structure with higher KER can be assigned as a combination of the two dissociation channels N2+ → N+ + N and N2++ → N+ + N+ (see also Fig. 3b), however our energy resolution is not sufficient to separate these channels. With reference to Fig. 3b, the part of peak E at lower binding energies (around 45 eV) can be assigned to the N2+ → N+ + N channel, and limits 11–13, which lead to N+ ions with inner-valence holes in the 2 s orbital. For binding energies above 50 eV, the broad diagonal structure is mainly due to the N2++ → N+ + N+ dissocation channel. The second, slow electron resulting from double Auger decay was not observed in the experiments, further reducing the resolution of the two-dimensional plots due to the breakdown of the energy sharing assumption.
Fig. 5. Two-dimensional Auger-electron−photoion (N+ or N2++) coincidence maps. The KER is obtained by doubling the kinetic energy of the detected N+ ions. The dashed diagonal lines correspond to the kinetic energies of the fragment ions as functions of the binding energy for specific dissociation limits, which are listed in Table 2. The pass energy is (a) 400 and (b) 100 eV. The spectrum of (c) is obtained by integrating the map of (a) in the vertical direction.
Table 2 Dissociation limits for forming fragment pairs of N+ + N [6,28]. The energies are calculated with the energy levels of N and N+ and the dissociation energy of N2→N + N. Label 1 2 3 4 5 6 7 8 9 10 11 12 13
State of fragments (N+, N) 3
4
( P, S°) (1D, 4S°) (3P, 2D°) (3P, 2P°) (1S, 4S°) (1D, 2D°) (1D, 2P°) (1S, 2D°)) (1S, 2P°) (3P, (3 s) 4P) ((2 s2p3) 3D°, 4S°) ((2 s2p3) 3P°, 4S°) ((2 s2p3)3D°, 2D°)
Dissociation limit energy (eV) 24.29 26.19 26.67 27.87 28.35 28.57 29.77 30.73 31.92 34.63 35.72 37.83 38.11
4. Conclusion In conclusion, we report extensive new information on the stability and dissociation dynamics of individual resonant Auger states produced following N 1s→π* core excitation of N2 molecules. From AEPICO coincidence measurements, we have derived coincident Auger spectra for both non-dissociative and dissociative cationic states. For dissociative states, two-dimensional AEPICO maps reveal the correlations between the binding energies of the Auger states and ion kinetic energy release. We have used this energy correlation information to assign dissociation limits to each Auger peak. By analyzing events where we observed either two N+ ions in coincidence or metastable N2++ ions, we also estimated the relative abundance of double Auger decay in the total 43
Journal of Electron Spectroscopy and Related Phenomena 232 (2019) 40–44
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Auger spectrum. Our results show that double Auger decay dominates the Auger spectra for binding energies greater than 50 eV.
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Resonant-Auger-state-selected dissociation dynamics and dissociation limits of N 1s→π* core excited N2 molecules studied using a two-dimensional Auger-electron-photoion coincidence method
T
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Hiroshi Iwayamaa,b, , James R Harriesc a
UVSOR Facility, Institute for Molecular Science, Nishigonaka 38, Myodaiji, Okazaki, 444-8585, Japan SOKENDAI, Nishigonaka 38, Myodaiji, Okazaki 444-8585, Japan c QST, SPring-8, Kouto 1-1-1, Sayo, Hyogo, 679-5148, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photodissociation Core excitation Auger-electron-photoion coincidence
The dissociation dynamics of core-excited N2 molecules has been investigated using a two-dimensional Auger electron photoion coincidence method, where the kinetic energies of Auger electrons and ions are measured in coincidence. From the energy correlations between the binding energies of the resonant Auger states and the ion kinetic energy release, the electronic states of fragments at the dissociation limit are investigated for each Auger peak. We also estimate the contribution of double Auger decay to the total Auger spectrum by analyzing events where two N+ ions or metastable doubly charged ions N2++ were detected.
1. Introduction The understanding of dissociation pathways is one of the most important topics in the study of the photoionization of molecules [1,2]. Since photoionization can lead to the production of a range of different cationic states, the subsequent relaxation and fragmentation process is generally complex. To understand the overall photodissociation process, it is thus important to reveal the links between the initial cationic states and the corresponding final products. Electron-ion coincidence measurements [3–9] have proved to be one of the best methods to investigate this state-selected dissociation dynamics. Photoionization in the vacuum ultraviolet (VUV) wavelength region has been widely used to study the dissociation processes of outer-valence ionized molecules. The groups of Nahon [3] and Sheng [4] have developed techniques for double-imaging photoelectron-photoion coincidence measurements, where electrons and ions are detected in coincidence using two velocity map imaging spectrometers. This allows the study of state-selected dissociation dynamics and the determination of dissociation limits, and provides information on not only the masses and charge states of fragment ions but also their electronic states. These advances have breathed new life into valence photoionization measurements, for example by providing new details on the mechanism by which the free halogen atoms F and Cl, which can deplete ozone in the stratosphere, are produced by the photodissociation of CH3X, for X = F [10], and X = Cl [11].
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The inner-valence photoionization of simple diatomic molecules has also been investigated using a high-resolution threshold photoelectronphotoion coincidence technique [5,6]. Since threshold photoelectrons have nearly zero kinetic energies, the binding energy of the initial cations is almost equal to the incident photon energy. Thus tuning the photon energy corresponds to selecting the initial cationic state. These high resolution measurements were successful in identifying the dissociation limits for some inner-valence ionic states, and the state-selected dissociation dynamics were compared to configuration interaction calculations. At higher photon energies, the absorption of soft x-rays can result in core excitation and inner-shell ionization of molecules [2]. For light elements, the dominant decay process of core-excited or core-ionized molecules is Auger decay. Similiarly to VUV photoionization, the initial ionic states can be deduced from the kinetic energies of the Auger electrons, and the Auger-electron-photoion coincidence (AEPICO) technique is widely used. However, in general, the Auger electrons emitted following soft X-ray absorption have high kinetic energies, making velocity momentum imaging and threshold photoelectron techniques difficult to apply. Morin et al. developed an AEPICO experimental setup combining a double toroidal electron analyzer and an ion momentum spectrometer [12,13]. The use of a double toroidal electron analyzer provides both high energy resolution and high detection efficiency for high energy electrons such as Auger electrons. The technique was used to analyze resonant-Auger-state-selected
Corresponding author. E-mail address: [email protected] (H. Iwayama).
https://doi.org/10.1016/j.elspec.2019.01.005 Received 14 November 2018; Received in revised form 14 January 2019; Accepted 23 January 2019 Available online 24 January 2019 0368-2048/ © 2019 Elsevier B.V. All rights reserved.
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dissociation dynamics, and has provided insight into the understanding of the relaxation dynamics following core excitation in molecules, such as ultrafast dissociation [14] and site selective dissociation [15,16]. We have developed a similar AEPICO spectrometer and recently reported on the stability and dissociation dynamics of core ionized N2 molecules [17]. Here, we extend this work, and present experimental results on the dissociation dynamics following 1s→π* excitation of N2. The results provide extensive information on both dissociative and nondissociative resonant Auger states. The AEPICO spectrometer enables us to measure the kinetic energies of Auger electrons and ions in coincidence for each photoionization event. By analysing the kinetic energies of the Auger electrons and product ions, we obtain two dimensional AEPICO maps showing the energy correlations between the binding energies of the resonant Auger states and the kinetic energy release (KER) of the fragments. This allows us to determine the dissociation limit for each resonant Auger state. In addition, we also estimate the relative abundance of double Auger decay in the total Auger spectrum.
Fig. 2. Total Auger electron spectrum following N 1s→π* excitation, recorded using a pass energy of 400 eV. State assignments for peaks labelled A – E are given in Table 1.
with a kinetic energy between 345 eV and 390 eV. Two peaks are observed, due mainly to parent ions N2+ and fragment ions N+. Most N+ fragment ions come from dissociation pathways N2+ → N+ + N. However it is also possible for core excited N2* molecules to undergo double Auger decay, ejecting two electrons, and leading to the pathway N2++ → N+ + N+. Further, metastable doubly charged parent ions N2++ ions, which can also be produced following double Auger decay, have the same mass to charge ratio as N+ ions. The contributions of double Auger decay are discussed further below. Fig. 2 shows the corresponding total Auger electron spectrum, recorded at a pass energy of 400 eV. Kinetic energy (bottom axis) and binding energy (top axis) scales are shown. The binding energy is calculated as hv – eKE, where hv is the photon energy (401.1 eV), and eKE is the electron kinetic energy. While the peak height at the kinetic energy of 385 eV is considerably lower due to the relatively low energy resolution (∼ 4 eV), spectral features are similar to those in previously reported resonant Auger spectra [21,22], and the peaks labelled A–E can be assigned to the main configurations listed in Table 1 [23]. The valence electronic configuration of the neutral ground state of N2 is (2σ g)2(2σ u)2(1π u)4(3σ g)2, where 2σ g is termed an inner-valence orbital, and 2σ u, 1π u, and 3σ g outer-valence orbitals. Peak A, at the lowest observed binding energy of around 17 eV, is assigned to the production of the X2Σ g+(3σ g−1) and A2Π u(1π u−1) states of N2+ by participator Auger decay. The production of the third and fourth lowest electronic states of N2+, B2Π g(3σ g-21π g1) and C2Σ u+(3σ g−11π u-1 1π g1) by spectator Auger decay are the main components of peak B. In addition, peak B also includes various electronic states originating from configurations of 3σ g-21π g1, 3σ g−11π u−11π g1 and 1π u-21π g1. Peak C is mainly due to states with a main configuration of 2σ u−11π u−11π g1, and peaks D and E are attributed to states with a hole in the innervalence orbital 2σ g. The use of resonant Auger decay has some advantages in producing inner-valence excited states, which are difficult to access via the direct valence photoionization because of their weak coupling to the direct ionization channel. In Fig. 3 we show the spectra for electrons detected in coincidence
2. Experimental setup The experiments were performed on the soft x-ray beamline BL6U at UVSOR in Japan. The radiation from an undulator was monochromatized by a variable-included-angle varied-line-spacing plane grating monochromator. The photon energy was set to 401.1 eV, which corresponds to N 1s→π* core excitation [18]. The experimental setup for AEPICO measurements comprised a double toroidal electron analyzer [12,13] and an ion momentum spectrometer, each of which is equipped with a time- and position-sensitive detector (RoentDek DLD40 [19]). The full details of the design and operation of the apparatus are described elsewhere [17,20]. The electron detector covered an energy range corresponding to 15% of the pass energy. For electron kinetic energies ranging from 345 to 390 eV, we performed AEPICO measurements at a pass energy of 400 eV, which corresponds to an energy resolution of about 4 eV. For electron kinetic energies ranging from 365 to 375 eV, we also made measurements with high electron energy resolution (about 1 eV), using a pass energy of 100 eV. When an electron was detected, a pulsed extraction field of 270 V/cm was applied to the photoionization region in order to extract ions towards the ion momentum spectrometer. The extraction field was high enough to collect all ions with kinetic energies of less than 6 eV. The detection efficiency of the ion detector was estimated to be about 40%. 3. Results and discussion Fig. 1 shows a time-of-flight spectrum for ions detected following the production of core excited N2*(1s−11πg) molecules at a photon energy of 401.1 eV, and in coincidence with the detection of an electron
Table 1 Peak assignments and main configurations of cationic states in the KVV Auger spectrum of core excited N2 [23].
Fig. 1. Ion time-of-flight spectrum recorded following N 1s→π* core excitation at a photon energy of 401.1 eV. This spectrum shows the times-of-flight of ions detected in coincidence with Auger electrons with kinetic energies ranging from 345 to 390 eV. 41
Label
Main configurations of N2+ states
A B C D E
3σ g−1, 1π u−1 3σ g−21π g1, 3σ g-11π u-11π g1, 1π u−21π g1 2σ u−11π u−11π g1 2σ g−1 2σ g−11π u−11π g1, 2σ g−12σ u−11π g1, 2σ g−13σ g−11π g1
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Fig. 4. Kinetic energy of N+ and N2++ ions above the binding energy of 42 eV.
corresponds to fragment N+ ions resulting from N2+ → N+ + N or N2++ → N+ + N+. Thus we can assume that ions corresponding to the sharp peak at 0 eV are metastable N2++ ions. The relative abundance of metastable N2++ is shown by the green filled area in Fig. 3b. The production of metastable N2++ ions is also clearly revealed in the twodimensional AEPICO maps presented below. The proportion of N+ ions resulting from N2++ → N+ + N+ dissociation can be estimated from the countrate of events where two N+ ions are detected in coincidence. To compare the event rates of electron-ion and electron-ion-ion coincidence detections, we need to take into account the ion detection efficiency and the detector dead time. The ion detection efficiency was estimated to be (40 ± 2) %, from the probability that N2+ ions were detected at a binding energy of 18 eV. To estimate the rate of false electron-ion-ion coincidence detections due to the ion detector’s dead time of 40 ns, we simulated the times-of-flight differences of two N+ fragment ions with kinetic energies of 4 ± 1 eV for 100,000 fragment pairs. The estimated false rate was (60 ± 6) %. The blue filled area in Fig. 3b represents the estimated contribution of N2++ → N+ + N+ events. The probability of double Auger decay is (19 ± 3) % for 1s→π* core excitation of N2 molecules. This value is much higher than that for N2 core ionization, which is (9.4 ± 0.9)% [26,27]. This difference may be explained by the fact that core excitation can follow spectator Auger decay, with binding energies that often exceed the double ionization threshold. To investigate the energy correlations between Auger electrons and fragment ions we present two-dimensional AEPICO maps in Fig. 5 for N2+ → N+ + N, where the vertical and horizontal axes represent the ion KER and the binding energy of the resonant Auger states, respectively. The KER plotted here corresponds to twice the kinetic energy determined for the detected N+ ions, since the two fragments have the same kinetic energy due to momentum conservation. The AEPICO maps allow us to discuss the dissociation limits of the Auger states corresponding to peaks B2, C, D and E shown in Fig. 3(b). Fig. 5(a) shows the AEPICO map covering the binding energy range from 20 to 55 eV, with the electrons detected using a pass energy of 400 eV (energy resolution ∼ 4 eV). Fig. 5(b) shows the AEPICO map corresponding to the high resolution (∼ 1 eV) data, recorded using a pass energy of 100 eV. Fig. 5(c) shows the spectrum obtained by integrating the AEPICO map of (a) in the vertical direction, which is same to the coincidence Auger spectrum for N+ ions in Fig. 3(b). The maps show several diagonal structures with a slope of 1, indicating strong correlations between Auger-electron binding energies and ion KERs. These energy correlations come from the energy conservation, KER = BE – DLE, where KER is the kinetic energy release of the fragments, BE is the binding energy of the resonant Auger states, and DLE is the dissociation limit energy. The dissociation limit energy corresponds to the binding energy at the dissociation asymptote, and depends on the electronic states of the atomic fragments as well as their charge states. The diagonal dashed lines in Fig. 5 indicate several dissociation limits. Possible dissociation limits for N2+ → N+ + N below 38 eV, obtained from Ref [6,28], are listed in Table 2. In Fig. 5(a), we also show the
Fig. 3. Coincidence Auger electron spectra for (a) parent N2+ ions and (b) fragment N+ and metastable N2++ions. The filled areas represent estimates of the contributions of N2++ → N+ + N+ (blue) and metastable N2++ ions (green) resulting from double Auger decay.
with N2+ (parent) ions (a), and fragment N+ ions (b). The energy of the lowest dissociation limit, N2+ → N+(3P) + N(4S), is also indicated. It is clear that this dissociation limit corresponds to a marked change in the coincidence Auger spectra. For binding energies below the lowest dissociation limit, only the parent ions, N2+, are detected. This can be associated with peak A, (X2Σ g+(3σ g−1) and A2Π u(1π u−1) states), and part of peak B. We label the non-dissociative and dissociative components of peak B peaks B1 and B2, respectively. The peak positions of B1 and B2 are 24 eV and 26 eV, and they can be mainly assigned to the electronic states B2Π g(3σ g-21π g1) (B1) and C2Σ u+(3σ g−11π g−11π g1) (B2). The stability of the B2Π g state (peak B1) with respect to dissociation is due to the binding energy (24.2 eV) being lower than the lowest dissociation limit energy (24.3 eV). Previous studies on outer valence photoionization [24] showed that, in addition to the B2Π g states, the C2Σ u+ state with v = 2, lying below the dissociation limit, is also stable, but that C2Σ u+with v = 3…7 dissociates into N2+ → N+ + N (states with v < 2 were not observed). While the vibrational structure of the C2Σ u+ state is not resolved in peak B, our results are consistent with the production of C2Σ u+ states mainly leading to dissociation. Above the lowest dissociation limit energy, the number of electrons detected in coincidence with parent ions N2+ rapidly decreases, with almost all of the created N2+ ions dissociating and producing N+ fragments. Below the N2 double ionization threshold of 42 eV [25], the counterpart to the N+ fragment is a neutral N atom, but above this threshold double Auger decay can also occur, leading to the production of N+ + N+ fragment pairs or metastable ions N2++. In our experiment we were unable to detect low energy electrons resulting from this double Auger decay, due to the high background signal from scattered electrons. The stray light of incident soft x-rays would hit electrodes and produce many satterted electrons. We discuss below the contributions from double Auger decay. Fig. 4 shows the kinetic energy distribution of ions with m/q = 14 amu/e− (N+ or N2++) detected above the double ionization threshold of 42 eV. A sharp peak can be seen 0 eV, and a broad peak at 4 eV. The sharp peak around 0 eV can be attributed to parent N2++ ions which are metastable and are detected before dissociation.The broad peak 42
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lowest dissociation limit of doubly charged ions, N2++ → N+(3P) + N+(3P), which lies at 38.8 eV [17]. First we discuss the dissociation pathways of the resonant Auger states corresponding to peak B2 of Fig. 5(c). The two-dimensional AEPICO map (Fig. 5b) reveals a diagonal island-like structure centred at a binding energy of around 26 eV. This peak corresponds to the C2Σ u+(3σ g−11π u−11π g1) states. The corresponding dissociation limit is the lowest dissociation limit 1 (N2+ → N+(3P) + N(4S°)). The diagonal island-like structure extends to a KER of 0 eV, showing that dissociation begins to occur once binding energies reach this limit. A further diagonal island-like structure can be seen also beginning at a kinetic energy release of 0 eV, with a binding energy of around 27 eV. The results suggest that these dissociative states are associated with dissociation limit 3 (N2+ → N+(3P) + N(2D°)) rather than dissociation limit 2 (N2+ → N+(1D) + N(4S°)). A third diagonal island-like structure can be seen in Fig. 5b, which can be associated with peak C in Fig. 5(c). While our electron resolution (∼ 1 eV) makes it difficult to completely separate this structure from that assigned above to states related to dissociation limit 3, this structure appears to be also associated with dissociation limit 4. For binding energies above 30 eV, the main dissociation limit for the resonant Auger states (2σ u−11π u−11π g1) which make up peak C is dissociation limit 4 (N2+ → N+(3P) + N(2P°)). Shifting our focus to Fig. 5(a), which shows data recorded with a pass energy of 400 eV (electron energy resolution ∼ 4 eV), we first discuss the dissociation dynamics of states with binding energies of around 36 eV. These resonant Auger states correspond to peak D of Fig. 5(c), and are electronic states with a hole in the inner-valence orbital 2σg. Diagonal structures at KERs of around 2 eV and 8 eV can be associated with these states, and lie along the dashed lines 7 (N2+ → N+(2p−1 1D) + N(2P°)) and 11 (N2+ → N+(2 s−1 3D°) + N(4S°)). This suggests the interpretaion that upon dissociation, the inner-valence hole in the molecular 2σg orbital becomes a hole in either the atomic 2 s or 2p orbital. This is understandable since the 2σg molecular orbital has its origins in a mixing of atomic 2 s and 2p orbitals. Finally we discuss the broad diagonal and horizontal structures which can be seen for binding energies of around 47 eV, corresponding to peak E in Fig. 5(c). In this energy region, the AEPICO map includes significant contributions from double Auger decay. The horizontal structure for KERs close to 0 eV can be assigned to metastable doublycharged parent ions, N2++. The broad diagonal structure with higher KER can be assigned as a combination of the two dissociation channels N2+ → N+ + N and N2++ → N+ + N+ (see also Fig. 3b), however our energy resolution is not sufficient to separate these channels. With reference to Fig. 3b, the part of peak E at lower binding energies (around 45 eV) can be assigned to the N2+ → N+ + N channel, and limits 11–13, which lead to N+ ions with inner-valence holes in the 2 s orbital. For binding energies above 50 eV, the broad diagonal structure is mainly due to the N2++ → N+ + N+ dissocation channel. The second, slow electron resulting from double Auger decay was not observed in the experiments, further reducing the resolution of the two-dimensional plots due to the breakdown of the energy sharing assumption.
Fig. 5. Two-dimensional Auger-electron−photoion (N+ or N2++) coincidence maps. The KER is obtained by doubling the kinetic energy of the detected N+ ions. The dashed diagonal lines correspond to the kinetic energies of the fragment ions as functions of the binding energy for specific dissociation limits, which are listed in Table 2. The pass energy is (a) 400 and (b) 100 eV. The spectrum of (c) is obtained by integrating the map of (a) in the vertical direction.
Table 2 Dissociation limits for forming fragment pairs of N+ + N [6,28]. The energies are calculated with the energy levels of N and N+ and the dissociation energy of N2→N + N. Label 1 2 3 4 5 6 7 8 9 10 11 12 13
State of fragments (N+, N) 3
4
( P, S°) (1D, 4S°) (3P, 2D°) (3P, 2P°) (1S, 4S°) (1D, 2D°) (1D, 2P°) (1S, 2D°)) (1S, 2P°) (3P, (3 s) 4P) ((2 s2p3) 3D°, 4S°) ((2 s2p3) 3P°, 4S°) ((2 s2p3)3D°, 2D°)
Dissociation limit energy (eV) 24.29 26.19 26.67 27.87 28.35 28.57 29.77 30.73 31.92 34.63 35.72 37.83 38.11
4. Conclusion In conclusion, we report extensive new information on the stability and dissociation dynamics of individual resonant Auger states produced following N 1s→π* core excitation of N2 molecules. From AEPICO coincidence measurements, we have derived coincident Auger spectra for both non-dissociative and dissociative cationic states. For dissociative states, two-dimensional AEPICO maps reveal the correlations between the binding energies of the Auger states and ion kinetic energy release. We have used this energy correlation information to assign dissociation limits to each Auger peak. By analyzing events where we observed either two N+ ions in coincidence or metastable N2++ ions, we also estimated the relative abundance of double Auger decay in the total 43
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Auger spectrum. Our results show that double Auger decay dominates the Auger spectra for binding energies greater than 50 eV.
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