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Long-time stability of superexcited high Rydberg molecular states
3 October 1997
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters 277 (1997) 147-152
Long-time stability of superexcited high Rydberg molecular states Lal A. Pinnaduwage a,b, Yifei Zhu a a Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6122, USA b Department of Physics, University of Tennessee, Knoxville, TN 37996-1200, USA
Received 1 July 1997;in final form 12 August 1997
Abstract
Using a time-resolved, mass analyzed, pulsed field ionization technique it is shown that molecules excited to energies within several electron volts above their lowest ionization thresholds can survive for several microseconds. These observations are consistent with the recent observations of efficient dissociative electron attachment to molecules excited to energies above their ionization thresholds. © 1997 Elsevier Science B.V.
1. Introduction When a molecule is excited to an energy above its ionization potential (IP), the ionization yield may not become unity for excitation energies up to a few eV above the lowest ionization threshold. Neutral molecular states lying above the lowest IP were given the name superexcited states (SES) by Platzman [1]. In molecules there is a natural mechanism to provide bound states that are isoenergetic with the continuum [2]: each ro-vibrational state of the ion core can support its own series of high Rydberg (HR) states. These SES may decay via several channels including dissociation and intramolecular coupling to stable states [3]. In the recent years, we have observed efficient electron attachment to molecules excited to energies above their IPs, see Refs. [4-6], and references therein. However, since the SES thus produced have been considered to be short-lived (with sub-nanosecond lifetimes), it was not clear how the attachment of an electron could occur. In this Letter, we show that molecules excited to crier-
gies above their IPs can have microsecond lifetimes. Thus, the present findings are consistent with the conclusions of the above-mentioned experiments [46] that electron attachment occurred to long-lived superexcited HR states, where the excited electron was in a high-Rydberg orbital around a core which carried the excess energy. The development of zero kinetic energy (ZEKE) photoelectron spectroscopy [7] and mass analyzed threshold ionization (MATI) spectroscopy [8] have simulated research on the dynamics of the HR molecular states as well as in ion spectroscopy. The Z E K E / M A T I techniques are based on the delayed pulsed field ionization (PFI) of the HR states photoexcited to an energy a few wave numbers (a few meV) below the ionization potential (IP); such HR states that are located close to the IP (with well-defined ion cores) are frequently called ZEKE states. It was shown that these states have lifetimes that are several orders of magnitude larger than the optically accessible low~/ (angular momentum) states. The longevity of these HR states has been recently con-
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-2614(97)00913-5
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L.A. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
firmed to be a consequence o f the h i g h - f (and h i g h - m / ) values possible for the high-n states [9,10] and this state-mixing has been proposed to be due to external fields and collisions [11,12]. Another proposed mechanism to explain the longevity o f the Z E K E states involves the breakdown o f the B o r n Oppenheimer approximation and intramolecular redistribution processes [2,13] leading to the mixing o f the different n (principal quantum number)-states as well as /-states. Most o f the " Z E K E / M A T I " experiments to date have been conducted on states hugging the low IPs [7], i.e. the ion cores were in their ground states or low ro-vibrational states. The longevity o f the " Z E K E states" excited to high vibrational states [14,15] o f the ion has recently been illustrated; in these studies, it was shown that a molecule excited to a regular Z E K E state can absorb an additional photon and still be stable for microsecond times under collisionless conditions. Our present studies generalize these observations: we present evidence to show that molecules excited to apparently arbitrary energies (within the linewidth = 150 cm - t o f the excimer laser used) well above their lowest IPs can have long lifetimes in the microsecond range. Our present experiments are different from the regular Z E K E experiments in several aspects: (i) we use an excimer laser with broad linewidth ( = 150 cm -n ) to excite the molecules, this is in contrast to the narrow linewidth dye lasers used to excite Z E K E states [7] with well-defined ion cores. (ii) The molecules are excited to energies > 0.7 eV above the IP and due to high density o f ro-vibrational states this will lead to strong vibronic coupling in the core. (iii) Normally, the Z E K E states are populated with > 100, but in the present case, some states we detect have 50 < n < 100, see below. The main objective o f the present experiments was to confirm the assertion o f the previous electron attachment studies [ 4 - 6 ] that the superexcited HR states o f molecules populated via excimer laser irradiation can have long enough lifetimes for electron attachment to occur. In the present experiments, we have excited benzene (IP -- 9.24 eV [16]), deuterated benzene (IP = 9.25 eV [17]) and triethylamine (IP = 7.5 eV [18]) to energies above their IPs, using both the KrF (wavel e n g t h - - 2 4 8 rim, photon energy = 5 eV) and A r F (193 nm, 6.4 eV) excimer laser lines via two-photon
excitation. In all six cases we observed the formation o f long-lived (lifetimes in the I~S range) superexcited HR states under collisionless conditions. In all six cases, the one-photon energy was above the first electronically-excited states (S t ) and thus efficient resonance-enhanced transitions to energies above the IPs were obtained.
2. E x p e r i m e n t a l Our experimental arrangement is shown in Fig. 1. The pulsed molecular beam was intercepted by a laser beam in the laser interaction region located between grids # 1 and # 2 . The unfocused laser beam was collimated with apertures and ~ 2 0 0 - 3 0 0 I~J of energy was delivered to the interaction region in an area with a radius o f ,-, 1 mm; under these conditions only the parent ion was produced via laser photoionization and no fragment ions were observed. The gas j e t velocity c a r d e d the positive ions and neutral excited species produced by the laser pulse to
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Fig. I. Schematic diagram of the experimental apparatus. Laser interaction region is located between the grids # 1 and #2 and the pulsed ionization/extraction region is located between the grids #2 and #3. At a negative or zero bias voltage on grid #2, the ions produced in the interaction region (direct ions) as well as the HR states are carried to the extraction region by the jet velocity. Propagation of the direct ions to the extraction region can be prevented by keeping the bias voltage on grid #2 at a sufficiently high positive value (the offset voltage on grid #3 is always kept at the same value as the bias voltage on grid #2). At a given time after the laser pulse, a positive voltage pulse is applied to grid #2 to ionize HR states and to extract all the positive ions present in the extraction region to the time of flight mass spectrometer.
L.A. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
the extraction region between the grids # 2 and #3. A voltage pulse of pulse height ranging from 70 to 120 V (pulse width of ~-20 Ws, pulse rise time 50 ns) was applied to grid # 2 after a given time delay from the laser pulse. This voltage pulse served two purposes: (i) it ionized the HR states present in the extraction region at that time, and (ii) it pushed out both the direct ions - - those produced via direct ionization in the interaction region - - and the ions produced in the extraction region via field ionization of the HRs, into a time-of-flight (TOF) mass spectrometer for mass analysis. The rising edge of this voltage pulse was taken to be the t = 0 for the flight time of the ions. The ions passing through grid # 3 were accelerated again by the - 800 V applied to the i m long TOF flight tube. The voltage on grid # 2 was kept at a bias voltage until the voltage pulse was applied. By keeping the positive bias voltage on grid # 2 above a certain threshold value, the direct ions could be prevented from entering the extraction region. Thus, the ion signal observed with a bias voltage above this cut-off value was due to the ionization of neutral HR states in the extraction region by the voltage pulse. Furthermore, we were able to separate the direct ions from the ions produced by the field ionization in the extraction region using the following method: the direct ions could be accelerated with respect to the HR states by applying a negative bias voltage on grid #2, i.e. the direct ions were accelerated in the interaction region and thus gained more energy compared to the neutral HR states. Therefore, the direct ions had smaller flight times in the TOF mass spectrometer even though the same ionic species (the parent ion) was detected in both cases, see below. The offset voltage on grid # 3 was maintained at the same voltage as the bias voltage on grid # 2 ; this offset voltage was not critical for the present experiments; hereafter, when we refer to a certain bias voltage on grid #2, it implies that the offset voltage on grid # 3 was kept at the same value. The pulsed valve was operated with low backing pressure ( 2 - 5 0 Torr) and without a carrier gas. Thus supersonic expansion did not occur and significant rotational cooling is not expected. The gas pulse was - - 2 0 0 I~s long and for a backing pressure of 10 Torr we estimate the pressure in the laser interaction region to be -~ 10 -6 Torr. We estimate the beam velocity
149
to have a broad distribution with average velocities around -- 400 to 500 ms -1. The ions were detected using a microchannel plate detector. No magnetic shielding was used in the interaction region. The laser was operated at a repetition rate of 1 Hz and each ion spectrum was obtained by averaging 32 laser shots. Excimer laser wavelengths at the KrF and ArF lines had pulse widths (FWHM) = 25 ns.
3. Results and discussion Data for benzene at the KrF laser line are shown in Fig. 2. These figures show the TOF spectra at different time delays of the ionization/extraction pulse with respect to the laser pulse; the pulsed voltage of 90 V used corresponds to a field ionized HR signal for states with n > 50. For time delays up to several ~s there was no signal because the direct i o n s / H R s took this time to move from the interaction region to the extraction region. The data taken for a zero bias voltage on grid # 2 are shown in Fig. 2a. The direct ions as well as the HRs moved with the same velocity distribution of the gas jet and hence the two signals were merged in this case. The signal profile was primarily determined by the velocity distribution of the gas jet. By having a positive bias voltage on grid #2, the direct ions could be decelerated in the interaction region and at a high enough bias voltage the direct ions were prevented from entering the extraction region. From the data of Fig. 2a, we estimate the fastest molecules from the gas jet to have velocities of -- 700 m s - 1 corresponding to an energy of -- 0.2 eV; they can be stopped by applying a bias voltage of -- 0.4 V to grid # 2 (the ions do not see the full potential drop). Fig. 2b shows data for + 1 V bias voltage applied to grid #2, where only the signals due to field-ionized HRs (with n < 150) were observed. It is clear from Fig. 2 that the neutral superexcited HR states have lifetimes exceeding 10 p,s; it is unlikely that any collisional stabilization could occur at the low backing pressure (-- 20 Tort) used for these data which is estimated to be = 10 -6 Tort. With decreasing backing pressure the overall signal strength was reduced, but the main features were still present down to backing pressures of = 2 Torr. For the data in Fig. 2c, the bias voltage on grid
L.A. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
150
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| Fig. 2. Time-of-fiight spectra of benzene at the KrF laser line and at different time delays of the ionization/extraction pulse with respect to the laser pulse. For these data the backing pressure, laser pulse energy, and the detector voltage were 19 Torr, 200 p.J, and - 1600 V respectively. (a) Data for zero bias voltage on grid # 2 . In this case, the signals due to the direct ions and the HRs overlap. The signal profile is determined by the both the finite size of the laser beam and by the velocity distribution of the gas jet. (b) Data for a bias voltage of + 1 V on grid # 2 . At this bias voltage the direct ions were completely stopped in the interaction region and only the signal due to the HRs that were field ionized in the extraction region was present. (c) Data for a bias voltage of - 5 V on grid # 2 . At this bias the direct ions were accelerated out of the interaction region and arrived early in the extraction region compared to the HR states. Furthermore, the direct ions started off with higher velocities and thus their times-of-flight were smaller compared to those for the ions resulted from field ionization in the extraction region.
# 2 was kept at - 5 V. In the presence of the - 5 V bias, the velocity distribution for the direct ions was compressed and was shifted to higher velocities due to the acceleration of the ions; this is the reason that the direct ions in Fig. 2c lasted for only -- l0 ~s compared to the combined direct i o n / H R signal of Fig. 2a which lasted for almost 40 p~s. The direct ions were accelerated out of the interaction region first (any HRs with n > 100 were also ionized by the bias field and were counted as direct ions.) Therefore, the direct ions arrived in the extraction region with higher velocities compared to the HRs (the ions also arrive early in the extraction region and thus the direct ion signal appears at a smaller delay time as well, see Fig. 2e); when the extraction voltage was applied, the direct ions started off with higher velocities than the ions produced via field ionization of the neutral HRs. The HRs are not accelerated by the bias field and thus the flight time for the HRs is the same as in the case of zero bias, see Fig. 2a. This separation of the HR and direct ion signals was reproduced by a computer simulation. At the KrF laser line, the benzene (and deuterated benzene) and the TEA molecules arc excited to energies of = 0.75 and = 2.5 eV above their respective lowest IPs; at the ArF laser line, the respective excess energies are -~ 3.55 and --5.3 eV respectively. In all cases we observed long-lived HR states. Generally, the HR to direct ion ratio decreased when the excess energy above the IP was increased. It must be noted that it is not possible to quantify the absolute signal strengths due to HRs and direct ions from these spectra. There is overlap in signals between the adjacent scans due to the finite laser beam size and the velocity spread of the gas jet. Fig. 3 shows data similar to those for Fig. 2c, but for different molecule/laser combinations; the detected HR states had 50 < n < 100, as before. For benzene at the ArF line, the signal was smaller than that at the KrF line, see Fig. 3a. Higher backing pressure, laser energy, and detector voltage were needed to obtain signals comparable to those of Fig. 2c. Furthermore, the direct ions signal as well as the HR signal did not last for as long as in Fig. 2c. All these observations could be due to the higher instability of the ion core at this higher excitation energy; the direct ion and the core of the HR will have quite similar internal energies and thus at the ArF line
hA. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
Fig. 3. Time-of-flight spectra for different molecule/laser combinations at different time delays of the ionization/extraction pulse with respect to the laser pulse, with - 5 V bias on grid #2. (a) Data for benzene/ArF. Backing pressure, laser pulse energy, and the detector voltage were 46 Torr, 300 It J, and - 1700 V respectively. (b) Data for triethylamine/KrF. Backing pressure, laser pulse energy, and the detector voltage were 6 Torr, 200 IxJ, and - 1 6 0 0 V respectively.
both will be fairly unstable compared to those excited at the KrF line. We did not observe any fragment ions, but for gradual fragmentation over extended times is not expected to yield a measurable signal at a given flight time. Data for triethylamine at the KrF line are shown in Fig. 3b and are qualitatively the same as those in Figs. 2c and 3a. It appears that the superexcited HR formation has quite common characteristics for different molecules, consistent with our electron attachment studies on various molecules (see Refs. [4-6], and references therein). Chupka [ 11,12] showed that the lifetimes of high Rydberg states are increased by /-mixing and m/mixing induced by external electric fields and collisions; these predictions have been confirmed by several studies [19-21]. Levine and coworkers (see Refs. [2,13], and references therein) have suggested that the lifetimes of the HRs are determined by the energy exchange between the Rydberg electron and
151
the ionic core. They argued that the lifetime enhancement of the ZEKE states at the sub-p.s scale is due to the /-mixing by the Stark effect, but the extreme stability in the ~s range is due to the "trapping effect" which arises from the high density of the bound HR states. Our experiments certainly explore a regime where a dense set of bound levels is coupled to a continuum: due to the many ro-vibrational states available to the core of a polyatomic molecule, there is a multitude of Rydberg series converging to these IPs (we also note that in the present experiments the molecules had a broad thermal population of all internal degrees of freedom). This congested bound level structure of the phase space of HR states and a narrow bottleneck for the exit to the continuum can be argued [2,13] to be the cause for the long time stability. We also note that the early experimental observation of "near-zero-energy" photoelectrons (see Refs. [22,23], and references therein) from molecules excited to arbitrary energies above their lowest IPs could have its origin in superexcited HR states.
4. Conclusions We have provided experimental evidence to show that molecules laser-excited to energies up to a few eV above their ionization thresholds can survive for microseconds; thus, in addition to direct ionization of the molecules, long-lived neutral Rydberg states are populated via laser excitation. This observation is consistent with the deduction from the previous electron attachment studies (see Refs. [4-6], and references therein), that superexcited high Rydberg states are responsible for the enhanced electron attachment observed in those experiments.
Acknowledgements We thank Dr. W.A. Chupka and Dr. S.H. Lin for fruitful discussions. This research is supported by the Environmental Management Science Program (EMSP) of the Department of Energy and by the National Science Foundation. The Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corporation for the US Department
152
of Energy under 96OR22464.
L.A. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
contract
number
DE-AC05-
References [1] R.L. Platzman, The Vortex 23 (1962) 372. [2] F. Remaele, U. Even, R.D. Levine, J. Phys. Chem. 100 (1996) 19735. [3] R.S. Berry, S. Leach, Adv. Electron. Electron Phys. 57 (1981) 1. [4] L.A. Pinnaduwage, P.G. Datskos, J. Appl. Phys. 81 (1997) 7715. [5] P.G. Datskos, L.A. Pinnaduwage, J.F. Kieikopf, Phys. Rev. A 55 (1997) 4131. [6] L.A. Pinnaduwage, D.L. McCorkle, Chem. Phys. Lett. 255 (1996) 410. [7] K. Miiller-Dethlefs, E.W. Schlag, E.R. Grant, K. Wang, B.V. McKoy, in: I. Pdgogine, S.A. Rice (Eds.), Advances in Chemical Physics, Wiley, New York, 1995, p. I. [8] L. Zhu, P. Johnson, J. Chem. Phys. 94 (1991) 5769.
[9] S.D. Chao, S.H. Lin, H.L. Selzle, E.W. Schlag, Chem. Phys. Lett. 265 (1997) 445. [I0] A. Held, Y.L. Baranov, H.L. Selzle, E.W. Schlag, J. Chem. Phys. 106 (1997) 6848. [I I] W.A. Chupka, J. Chem. Phys. 98 (1993) 4520. [12] W.A. Chupka, J. Chem. Phys. 99 (1993) 5800. [13] F. Remade, R.D. Levine, J. Chem. Phys. 105 (1996) 4649. [14] H. Krause, H.J. Neusser, J. Chem. Phys. 99 (1993) 6278. [15] W.G. Scherzer, H.L. Selzle, E.W. Schlag, R.D. Levine, Phys. Rev. Left. 72 (1994) 1435. [16] L.A. Chewter, M. Sander, K. Miiller-Dethlefs, E.W. Schlag, J. Chem. Phys. 86 (1986) 4737. [17] H.M. Rosenstock, K. Draxl, B.W. Steiner, J.T. Hen'on, J. Phys. Chem. Ref. Data 6 ((Suppl. I)) (1977) II. [18] K. Watanabe, T. Nakayama, J. Mottl, J. Quant. Spectrosc. Radiat. Transf. 2 (1962) 369. [19] G. Reiser, W. Habenicht, K. Mtiller-Dethlefs, E.W. Schlag, Chem. Phys. Lett. 152 (1988) 119. [20] S.T. Pratt, J. Chem. Phys. 98 (1993) 9241. [21] F. Merkt, R.N. Zare, J. Chem. Phys. I01 (1994) 3495. [22] T. Baer, P.-M. Guyon, I. Nenner, A. Tabche-Fouhaille, R. Botter, L.F.A. Ferreira, T.R. Govers, J. Chem. Phys. 70 (1979) 1585. [23] J.C. Miller, R.N. Compton, J. Chem. Phys. 75 (1981) 22.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters 277 (1997) 147-152
Long-time stability of superexcited high Rydberg molecular states Lal A. Pinnaduwage a,b, Yifei Zhu a a Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6122, USA b Department of Physics, University of Tennessee, Knoxville, TN 37996-1200, USA
Received 1 July 1997;in final form 12 August 1997
Abstract
Using a time-resolved, mass analyzed, pulsed field ionization technique it is shown that molecules excited to energies within several electron volts above their lowest ionization thresholds can survive for several microseconds. These observations are consistent with the recent observations of efficient dissociative electron attachment to molecules excited to energies above their ionization thresholds. © 1997 Elsevier Science B.V.
1. Introduction When a molecule is excited to an energy above its ionization potential (IP), the ionization yield may not become unity for excitation energies up to a few eV above the lowest ionization threshold. Neutral molecular states lying above the lowest IP were given the name superexcited states (SES) by Platzman [1]. In molecules there is a natural mechanism to provide bound states that are isoenergetic with the continuum [2]: each ro-vibrational state of the ion core can support its own series of high Rydberg (HR) states. These SES may decay via several channels including dissociation and intramolecular coupling to stable states [3]. In the recent years, we have observed efficient electron attachment to molecules excited to energies above their IPs, see Refs. [4-6], and references therein. However, since the SES thus produced have been considered to be short-lived (with sub-nanosecond lifetimes), it was not clear how the attachment of an electron could occur. In this Letter, we show that molecules excited to crier-
gies above their IPs can have microsecond lifetimes. Thus, the present findings are consistent with the conclusions of the above-mentioned experiments [46] that electron attachment occurred to long-lived superexcited HR states, where the excited electron was in a high-Rydberg orbital around a core which carried the excess energy. The development of zero kinetic energy (ZEKE) photoelectron spectroscopy [7] and mass analyzed threshold ionization (MATI) spectroscopy [8] have simulated research on the dynamics of the HR molecular states as well as in ion spectroscopy. The Z E K E / M A T I techniques are based on the delayed pulsed field ionization (PFI) of the HR states photoexcited to an energy a few wave numbers (a few meV) below the ionization potential (IP); such HR states that are located close to the IP (with well-defined ion cores) are frequently called ZEKE states. It was shown that these states have lifetimes that are several orders of magnitude larger than the optically accessible low~/ (angular momentum) states. The longevity of these HR states has been recently con-
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0009-2614(97)00913-5
148
L.A. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
firmed to be a consequence o f the h i g h - f (and h i g h - m / ) values possible for the high-n states [9,10] and this state-mixing has been proposed to be due to external fields and collisions [11,12]. Another proposed mechanism to explain the longevity o f the Z E K E states involves the breakdown o f the B o r n Oppenheimer approximation and intramolecular redistribution processes [2,13] leading to the mixing o f the different n (principal quantum number)-states as well as /-states. Most o f the " Z E K E / M A T I " experiments to date have been conducted on states hugging the low IPs [7], i.e. the ion cores were in their ground states or low ro-vibrational states. The longevity o f the " Z E K E states" excited to high vibrational states [14,15] o f the ion has recently been illustrated; in these studies, it was shown that a molecule excited to a regular Z E K E state can absorb an additional photon and still be stable for microsecond times under collisionless conditions. Our present studies generalize these observations: we present evidence to show that molecules excited to apparently arbitrary energies (within the linewidth = 150 cm - t o f the excimer laser used) well above their lowest IPs can have long lifetimes in the microsecond range. Our present experiments are different from the regular Z E K E experiments in several aspects: (i) we use an excimer laser with broad linewidth ( = 150 cm -n ) to excite the molecules, this is in contrast to the narrow linewidth dye lasers used to excite Z E K E states [7] with well-defined ion cores. (ii) The molecules are excited to energies > 0.7 eV above the IP and due to high density o f ro-vibrational states this will lead to strong vibronic coupling in the core. (iii) Normally, the Z E K E states are populated with > 100, but in the present case, some states we detect have 50 < n < 100, see below. The main objective o f the present experiments was to confirm the assertion o f the previous electron attachment studies [ 4 - 6 ] that the superexcited HR states o f molecules populated via excimer laser irradiation can have long enough lifetimes for electron attachment to occur. In the present experiments, we have excited benzene (IP -- 9.24 eV [16]), deuterated benzene (IP = 9.25 eV [17]) and triethylamine (IP = 7.5 eV [18]) to energies above their IPs, using both the KrF (wavel e n g t h - - 2 4 8 rim, photon energy = 5 eV) and A r F (193 nm, 6.4 eV) excimer laser lines via two-photon
excitation. In all six cases we observed the formation o f long-lived (lifetimes in the I~S range) superexcited HR states under collisionless conditions. In all six cases, the one-photon energy was above the first electronically-excited states (S t ) and thus efficient resonance-enhanced transitions to energies above the IPs were obtained.
2. E x p e r i m e n t a l Our experimental arrangement is shown in Fig. 1. The pulsed molecular beam was intercepted by a laser beam in the laser interaction region located between grids # 1 and # 2 . The unfocused laser beam was collimated with apertures and ~ 2 0 0 - 3 0 0 I~J of energy was delivered to the interaction region in an area with a radius o f ,-, 1 mm; under these conditions only the parent ion was produced via laser photoionization and no fragment ions were observed. The gas j e t velocity c a r d e d the positive ions and neutral excited species produced by the laser pulse to
-4
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13
0 0.5
2.3
3.3
~ Z(cm)
II I I I G'idl
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C~'id3
[ o [ Iaee~ bema
TOf 1
l
!
j ~ _ Otet F~I~tube ~olu~
Volu~
= - 800V
,,oUnce t
Fig. I. Schematic diagram of the experimental apparatus. Laser interaction region is located between the grids # 1 and #2 and the pulsed ionization/extraction region is located between the grids #2 and #3. At a negative or zero bias voltage on grid #2, the ions produced in the interaction region (direct ions) as well as the HR states are carried to the extraction region by the jet velocity. Propagation of the direct ions to the extraction region can be prevented by keeping the bias voltage on grid #2 at a sufficiently high positive value (the offset voltage on grid #3 is always kept at the same value as the bias voltage on grid #2). At a given time after the laser pulse, a positive voltage pulse is applied to grid #2 to ionize HR states and to extract all the positive ions present in the extraction region to the time of flight mass spectrometer.
L.A. Pinnaduwage, Y. Zhu / Chemical Physics Letters 277 (1997) 147-152
the extraction region between the grids # 2 and #3. A voltage pulse of pulse height ranging from 70 to 120 V (pulse width of ~-20 Ws, pulse rise time 50 ns) was applied to grid # 2 after a given time delay from the laser pulse. This voltage pulse served two purposes: (i) it ionized the HR states present in the extraction region at that time, and (ii) it pushed out both the direct ions - - those produced via direct ionization in the interaction region - - and the ions produced in the extraction region via field ionization of the HRs, into a time-of-flight (TOF) mass spectrometer for mass analysis. The rising edge of this voltage pulse was taken to be the t = 0 for the flight time of the ions. The ions passing through grid # 3 were accelerated again by the - 800 V applied to the i m long TOF flight tube. The voltage on grid # 2 was kept at a bias voltage until the voltage pulse was applied. By keeping the positive bias voltage on grid # 2 above a certain threshold value, the direct ions could be prevented from entering the extraction region. Thus, the ion signal observed with a bias voltage above this cut-off value was due to the ionization of neutral HR states in the extraction region by the voltage pulse. Furthermore, we were able to separate the direct ions from the ions produced by the field ionization in the extraction region using the following method: the direct ions could be accelerated with respect to the HR states by applying a negative bias voltage on grid #2, i.e. the direct ions were accelerated in the interaction region and thus gained more energy compared to the neutral HR states. Therefore, the direct ions had smaller flight times in the TOF mass spectrometer even though the same ionic species (the parent ion) was detected in both cases, see below. The offset voltage on grid # 3 was maintained at the same voltage as the bias voltage on grid # 2 ; this offset voltage was not critical for the present experiments; hereafter, when we refer to a certain bias voltage on grid #2, it implies that the offset voltage on grid # 3 was kept at the same value. The pulsed valve was operated with low backing pressure ( 2 - 5 0 Torr) and without a carrier gas. Thus supersonic expansion did not occur and significant rotational cooling is not expected. The gas pulse was - - 2 0 0 I~s long and for a backing pressure of 10 Torr we estimate the pressure in the laser interaction region to be -~ 10 -6 Torr. We estimate the beam velocity
149
to have a broad distribution with average velocities around -- 400 to 500 ms -1. The ions were detected using a microchannel plate detector. No magnetic shielding was used in the interaction region. The laser was operated at a repetition rate of 1 Hz and each ion spectrum was obtained by averaging 32 laser shots. Excimer laser wavelengths at the KrF and ArF lines had pulse widths (FWHM) = 25 ns.
3. Results and discussion Data for benzene at the KrF laser line are shown in Fig. 2. These figures show the TOF spectra at different time delays of the ionization/extraction pulse with respect to the laser pulse; the pulsed voltage of 90 V used corresponds to a field ionized HR signal for states with n > 50. For time delays up to several ~s there was no signal because the direct i o n s / H R s took this time to move from the interaction region to the extraction region. The data taken for a zero bias voltage on grid # 2 are shown in Fig. 2a. The direct ions as well as the HRs moved with the same velocity distribution of the gas jet and hence the two signals were merged in this case. The signal profile was primarily determined by the velocity distribution of the gas jet. By having a positive bias voltage on grid #2, the direct ions could be decelerated in the interaction region and at a high enough bias voltage the direct ions were prevented from entering the extraction region. From the data of Fig. 2a, we estimate the fastest molecules from the gas jet to have velocities of -- 700 m s - 1 corresponding to an energy of -- 0.2 eV; they can be stopped by applying a bias voltage of -- 0.4 V to grid # 2 (the ions do not see the full potential drop). Fig. 2b shows data for + 1 V bias voltage applied to grid #2, where only the signals due to field-ionized HRs (with n < 150) were observed. It is clear from Fig. 2 that the neutral superexcited HR states have lifetimes exceeding 10 p,s; it is unlikely that any collisional stabilization could occur at the low backing pressure (-- 20 Tort) used for these data which is estimated to be = 10 -6 Tort. With decreasing backing pressure the overall signal strength was reduced, but the main features were still present down to backing pressures of = 2 Torr. For the data in Fig. 2c, the bias voltage on grid
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]
nlr~t Ion. Hne
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E
/
| Fig. 2. Time-of-fiight spectra of benzene at the KrF laser line and at different time delays of the ionization/extraction pulse with respect to the laser pulse. For these data the backing pressure, laser pulse energy, and the detector voltage were 19 Torr, 200 p.J, and - 1600 V respectively. (a) Data for zero bias voltage on grid # 2 . In this case, the signals due to the direct ions and the HRs overlap. The signal profile is determined by the both the finite size of the laser beam and by the velocity distribution of the gas jet. (b) Data for a bias voltage of + 1 V on grid # 2 . At this bias voltage the direct ions were completely stopped in the interaction region and only the signal due to the HRs that were field ionized in the extraction region was present. (c) Data for a bias voltage of - 5 V on grid # 2 . At this bias the direct ions were accelerated out of the interaction region and arrived early in the extraction region compared to the HR states. Furthermore, the direct ions started off with higher velocities and thus their times-of-flight were smaller compared to those for the ions resulted from field ionization in the extraction region.
# 2 was kept at - 5 V. In the presence of the - 5 V bias, the velocity distribution for the direct ions was compressed and was shifted to higher velocities due to the acceleration of the ions; this is the reason that the direct ions in Fig. 2c lasted for only -- l0 ~s compared to the combined direct i o n / H R signal of Fig. 2a which lasted for almost 40 p~s. The direct ions were accelerated out of the interaction region first (any HRs with n > 100 were also ionized by the bias field and were counted as direct ions.) Therefore, the direct ions arrived in the extraction region with higher velocities compared to the HRs (the ions also arrive early in the extraction region and thus the direct ion signal appears at a smaller delay time as well, see Fig. 2e); when the extraction voltage was applied, the direct ions started off with higher velocities than the ions produced via field ionization of the neutral HRs. The HRs are not accelerated by the bias field and thus the flight time for the HRs is the same as in the case of zero bias, see Fig. 2a. This separation of the HR and direct ion signals was reproduced by a computer simulation. At the KrF laser line, the benzene (and deuterated benzene) and the TEA molecules arc excited to energies of = 0.75 and = 2.5 eV above their respective lowest IPs; at the ArF laser line, the respective excess energies are -~ 3.55 and --5.3 eV respectively. In all cases we observed long-lived HR states. Generally, the HR to direct ion ratio decreased when the excess energy above the IP was increased. It must be noted that it is not possible to quantify the absolute signal strengths due to HRs and direct ions from these spectra. There is overlap in signals between the adjacent scans due to the finite laser beam size and the velocity spread of the gas jet. Fig. 3 shows data similar to those for Fig. 2c, but for different molecule/laser combinations; the detected HR states had 50 < n < 100, as before. For benzene at the ArF line, the signal was smaller than that at the KrF line, see Fig. 3a. Higher backing pressure, laser energy, and detector voltage were needed to obtain signals comparable to those of Fig. 2c. Furthermore, the direct ions signal as well as the HR signal did not last for as long as in Fig. 2c. All these observations could be due to the higher instability of the ion core at this higher excitation energy; the direct ion and the core of the HR will have quite similar internal energies and thus at the ArF line
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Fig. 3. Time-of-flight spectra for different molecule/laser combinations at different time delays of the ionization/extraction pulse with respect to the laser pulse, with - 5 V bias on grid #2. (a) Data for benzene/ArF. Backing pressure, laser pulse energy, and the detector voltage were 46 Torr, 300 It J, and - 1700 V respectively. (b) Data for triethylamine/KrF. Backing pressure, laser pulse energy, and the detector voltage were 6 Torr, 200 IxJ, and - 1 6 0 0 V respectively.
both will be fairly unstable compared to those excited at the KrF line. We did not observe any fragment ions, but for gradual fragmentation over extended times is not expected to yield a measurable signal at a given flight time. Data for triethylamine at the KrF line are shown in Fig. 3b and are qualitatively the same as those in Figs. 2c and 3a. It appears that the superexcited HR formation has quite common characteristics for different molecules, consistent with our electron attachment studies on various molecules (see Refs. [4-6], and references therein). Chupka [ 11,12] showed that the lifetimes of high Rydberg states are increased by /-mixing and m/mixing induced by external electric fields and collisions; these predictions have been confirmed by several studies [19-21]. Levine and coworkers (see Refs. [2,13], and references therein) have suggested that the lifetimes of the HRs are determined by the energy exchange between the Rydberg electron and
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the ionic core. They argued that the lifetime enhancement of the ZEKE states at the sub-p.s scale is due to the /-mixing by the Stark effect, but the extreme stability in the ~s range is due to the "trapping effect" which arises from the high density of the bound HR states. Our experiments certainly explore a regime where a dense set of bound levels is coupled to a continuum: due to the many ro-vibrational states available to the core of a polyatomic molecule, there is a multitude of Rydberg series converging to these IPs (we also note that in the present experiments the molecules had a broad thermal population of all internal degrees of freedom). This congested bound level structure of the phase space of HR states and a narrow bottleneck for the exit to the continuum can be argued [2,13] to be the cause for the long time stability. We also note that the early experimental observation of "near-zero-energy" photoelectrons (see Refs. [22,23], and references therein) from molecules excited to arbitrary energies above their lowest IPs could have its origin in superexcited HR states.
4. Conclusions We have provided experimental evidence to show that molecules laser-excited to energies up to a few eV above their ionization thresholds can survive for microseconds; thus, in addition to direct ionization of the molecules, long-lived neutral Rydberg states are populated via laser excitation. This observation is consistent with the deduction from the previous electron attachment studies (see Refs. [4-6], and references therein), that superexcited high Rydberg states are responsible for the enhanced electron attachment observed in those experiments.
Acknowledgements We thank Dr. W.A. Chupka and Dr. S.H. Lin for fruitful discussions. This research is supported by the Environmental Management Science Program (EMSP) of the Department of Energy and by the National Science Foundation. The Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corporation for the US Department
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of Energy under 96OR22464.
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References [1] R.L. Platzman, The Vortex 23 (1962) 372. [2] F. Remaele, U. Even, R.D. Levine, J. Phys. Chem. 100 (1996) 19735. [3] R.S. Berry, S. Leach, Adv. Electron. Electron Phys. 57 (1981) 1. [4] L.A. Pinnaduwage, P.G. Datskos, J. Appl. Phys. 81 (1997) 7715. [5] P.G. Datskos, L.A. Pinnaduwage, J.F. Kieikopf, Phys. Rev. A 55 (1997) 4131. [6] L.A. Pinnaduwage, D.L. McCorkle, Chem. Phys. Lett. 255 (1996) 410. [7] K. Miiller-Dethlefs, E.W. Schlag, E.R. Grant, K. Wang, B.V. McKoy, in: I. Pdgogine, S.A. Rice (Eds.), Advances in Chemical Physics, Wiley, New York, 1995, p. I. [8] L. Zhu, P. Johnson, J. Chem. Phys. 94 (1991) 5769.
[9] S.D. Chao, S.H. Lin, H.L. Selzle, E.W. Schlag, Chem. Phys. Lett. 265 (1997) 445. [I0] A. Held, Y.L. Baranov, H.L. Selzle, E.W. Schlag, J. Chem. Phys. 106 (1997) 6848. [I I] W.A. Chupka, J. Chem. Phys. 98 (1993) 4520. [12] W.A. Chupka, J. Chem. Phys. 99 (1993) 5800. [13] F. Remade, R.D. Levine, J. Chem. Phys. 105 (1996) 4649. [14] H. Krause, H.J. Neusser, J. Chem. Phys. 99 (1993) 6278. [15] W.G. Scherzer, H.L. Selzle, E.W. Schlag, R.D. Levine, Phys. Rev. Left. 72 (1994) 1435. [16] L.A. Chewter, M. Sander, K. Miiller-Dethlefs, E.W. Schlag, J. Chem. Phys. 86 (1986) 4737. [17] H.M. Rosenstock, K. Draxl, B.W. Steiner, J.T. Hen'on, J. Phys. Chem. Ref. Data 6 ((Suppl. I)) (1977) II. [18] K. Watanabe, T. Nakayama, J. Mottl, J. Quant. Spectrosc. Radiat. Transf. 2 (1962) 369. [19] G. Reiser, W. Habenicht, K. Mtiller-Dethlefs, E.W. Schlag, Chem. Phys. Lett. 152 (1988) 119. [20] S.T. Pratt, J. Chem. Phys. 98 (1993) 9241. [21] F. Merkt, R.N. Zare, J. Chem. Phys. I01 (1994) 3495. [22] T. Baer, P.-M. Guyon, I. Nenner, A. Tabche-Fouhaille, R. Botter, L.F.A. Ferreira, T.R. Govers, J. Chem. Phys. 70 (1979) 1585. [23] J.C. Miller, R.N. Compton, J. Chem. Phys. 75 (1981) 22.