Letter pubs.acs.org/JPCL
A Novel Method to Measure Electronic Spectra of Cold Molecular Ions Satrajit Chakrabarty,† Mathias Holz,† Ewen K. Campbell,† Agniva Banerjee,† Dieter Gerlich,†,‡ and John P. Maier*,† †
University of Basel, CH-4056 Basel, Switzerland University of Technology, 09107 Chemnitz, Germany
‡
ABSTRACT: A universal method has been developed for measuring spectra of molecular ions in a 22-pole radio frequency trap at 5 K. It is based on laser induced inhibition of complex growth (LIICG). The first successful measurements have been demonstrated on the A2Πu ← X2Σ+g electronic transition with some thousand N+2 ions, helium densities of 1015 cm−3, and storage times of 1 s. The reduction of the number of N+2 −He complexes is the result of an interplay between excitation, radiative and collisional cooling, ternary association, and collision induced dissociation, which is explained by a kinetic model.
SECTION: Spectroscopy, Photochemistry, and Excited States
A
temperatures. Collisions with para-H2 have been used for recording rotational spectra of H2D+ and D2H+.10 Another example is the endothermic proton transfer from CH+5 to CO2, which has been used to obtain a vibrational spectrum of this ion.11 Such approaches; however, are limited in their applicability, mainly because of condensation of the probing molecules. In consideration of these requirements, a more general approach has been developed to measure the electronic absorption spectra of cold cations that does not rely on fragmentation or specific chemical reactions. It is based on ion confinement in a temperature variable trap by inhomogeneous rf fields. The present work pushes the boundary of conventional trapping technology by employing temperatures down to a few K and helium buffer gas density up to 1016 cm−3, resulting in ion mean free paths much smaller than the trap diameter. Under such conditions, helium attaches to all ions via ternary collisions, sooner or later. The association process is impeded by excitation of the molecular ion, allowing measurement of its absorption spectrum. The first application of this new technique is presented here, demonstrated on N+2 . Production of weakly bound complexes in supersonic beams, also in ion spectroscopy, has been used.12 The method of selective heating with an IR laser has been used in isotope separation by selective inhibition of condensation with argon.13
number of methods have been developed to measure electronic spectra of polyatomic molecular cations. Early techniques relied on fluorescence from excited electronic states. However, as the majority of larger cations do not fluoresce, alternative methods were sought.1 Some success was achieved by pulsed and continuous wave (cw) cavity ring down on ions generated in discharge sources coupled with supersonic expansion. This method suffers from the drawback that the identification relies on a detailed spectroscopic analysis because a myriad of species is produced concomitantly. On the other hand, techniques using mass selection have the advantage that the m/z is known. Furthermore, in order to compare laboratory spectra with astronomical observations in the visible, the vibrational and rotational degrees of freedom should be equilibrated to the low temperatures, ≈ 3−80 K, of the interstellar environment. This relaxation is not always achieved in a supersonic expansion of large molecules. Unique features of ion traps, long storage times, laser cooling, and sequential probing of the ion cloud with collisions or photons lead to outstanding sensitivities and allow spectroscopic measurements on less than 1000 ions. A variety of schemes have been developed for monitoring the absorption of a photon, e.g., laser induced fluorescence2−4 or fragmentation.5−7 In the case of larger molecular ions, internal conversion can be fast (sub-picoseconds) and fragmentation rates can be very slow. A class of detection schemes is based on changes of the reactivity of stored ions after their excitation.8 An early application of a low temperature radio frequency (rf) ion trap was the laser induced charge transfer (LICT) from argon to N2+.9 Laser-induced H−D exchange in the endothermic direction is a rather general approach at low enough © 2013 American Chemical Society
Received: October 18, 2013 Accepted: November 7, 2013 Published: November 7, 2013 4051
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formation of N+2 −He while the simultaneous addition of internal energy to N+2 via resonant excitation hinders this. The two competing processes together with collisional destruction of complexes and other mechanisms, result in a stationary equilibrium in the trapped ion ensemble. The most important elementary steps are indicated in Figure 2. Although the
The central part of the apparatus is shown in Figure 1. N+2 ions were produced by electron bombardment of the neutral
Figure 2. Competing processes leading to the measured signal. N+2 * ions, injected into the trap, relax in collisions with 5.5 K helium buffer gas. Especially fast is the rotational relaxation (ΔN = 2). Cold ions finally form N+2 −He complexes via ternary association (rate coefficient k3). Collision induced dissociation recycles them with a rate coefficient kCID. A significant fraction of N+2 ions, promoted with the laser (hν) to the A2Πu state, radiates with the rate krad to the excited vibrational levels of the X2Σ+g state (N+2 *). This reduces the formation of complexes. Figure 1. Central part of the experimental setup and timing sequence. An rf operated 22-pole trap is filled for 10 ms with some thousand N+2 . During the first 500 ms, they interact with high density helium buffer gas, leading to formation of up to 5% of N+2 −He complexes. For LIICG, the laser light interacts with the ion cloud during the same time. The trap is opened, and the content is analyzed using a mass spectrometer just before the next cycle starts.
stationary population of N+2 −He is only 4−5%, this proved sufficient for recording the electronic spectrum. Excitation of the N+2 ions with 10 mW from a cw Ti:Sa ring laser with 0.5 MHz bandwidth leads to a reduction of this number by a significant fraction. Figure 3 shows a section of the N+2 A2Πu ← X2Σ+g (2, 0) band. The absolute number of N+2 −He complexes per filling is plotted
gas. Following mass selection, they were injected into a 22-pole ion trap, where they were confined by an inhomogeneous rf field (amplitude ≈ 20 V; frequency ≈ 5 MHz). The trap is mounted on the second stage of a closed cycle helium cryostat. The measurements have been carried out around 5 K. This is the temperature of the trap walls, as well as that of helium, THe. As illustrated in Figure 1, the trap is loaded with ions during an injection period of 10 ms. The translational and rotational degrees of freedom of the ions were cooled in collisions with helium in less than 1 ms, while vibrational relaxation is slower. The buffer gas was leaked into the trap continuously for 500 ms by resonantly exciting the piezo actuator driven valve with a function generator (3 kHz, 10 V). The trap contents were extracted and analyzed 480 ms later when the helium pressure had decreased by orders of magnitude. This ensures increased detection efficiency of the primary ions and avoids collision induced fragmentation of weakly bound products. A capacitance manometer, directly connected to the copper box surrounding the trap, measured the absolute pressure. Thermal transpiration has to be accounted for, as the manometer operates at room temperature while the trap is held at a few Kelvin. The density is calculated from the pressure, using the correction function for helium empirically determined in ref 14. For an operating pressure of 3 × 10−4 mbar in the vacuum chamber containing the trap (pumping speed ≈ 430 l/s) and T = 5.5 K, [He] = 4.8 × 1015 cm−3 is obtained. The timing sequence in Figure 1 indicates that the laser irradiates the N+2 ions at the same time while they interact with the high density of helium. Collisions with helium lead to the
Figure 3. Spectrum of the A2Πu ← X2Σ+g (2,0) transition of N+2 recorded via the decrease of the number of N+2 −He complexes.
as a function of the laser wavelength. Each point is averaged over 10 iterations. To remove long time fluctuations in the ion beam or helium density, attempts were made to obtain a reference signal by alternatively measuring with and without laser. This, however, increased the instability of the signal, primarily due to laser induced thermal fluctuations of the entrance and exit electrode of the trap. The observed rotational transitions arise out of the electronic fine structure components F1(J = N + 1/2) and F2(J = N − 1/2) of the N+2 ground state in its two nuclear spin components ortho-N+2 (N = 0, 2, 4...) and para-N+2 (N = 1, 3, 5...). Spin splitting in the electronic ground state leads to the doublet structure. It is well resolved for the shown transitions with the exception of the two lines Q11(1.5) 4052
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and R12(0.5). They are separated by 0.014 cm−1. The nominal Doppler width at 5.5 K is 0.004 cm−1 (fwhm) corresponding to 121 MHz, while analysis of several line profiles leads to (350 ± 40) MHz. To exemplify, the R11(0.5) line, measured by LIICG, is shown in the upper part of Figure 4. Assuming that
Inspection of Figure 3 reveals that the changes in the number of N+2 −He complexes are not simply related to the population of the rotational states, as can be seen in the second column in Table 1. To calculate the experimental depletion, one has to Table 1. Comparison of the Calculated Thermal Population of the Rotational States at Trot = 10.6 K with the Depletion (in %) Derived from Measurementsa
a
N
population
depletion
0 1 2 3 4
31.8 28.4 33.3 4.9 1.6
37 26 32 19
See Figure 3 and text.
consider that ortho and para ions are separated ensembles in the trap since nuclear spin changing transitions via photons or collisions are strongly forbidden. The population ratio from the high temperature ion source, ortho:para = 2:1, is conserved. Surprisingly, efficient suppression of N+2 −He formation can be achieved by pumping from the N = 4 rotational level in the X2Σg+ state, although its population is only 1.6%. The explanation is that this population, depleted with the laser, is refilled readily due to fast rotational relaxation. In order to quantitatively understand how the laser induced signal is generated, the interactions of the ions with helium and laser have been simulated. For simplicity, the ion cloud is composed of ortho-N+2 (N = 0, 2, 4, 6), N+2 −He products and N+2 * (“hot” ions). Electronic excitation to the A2Πu state (Figure 2) is not explicitly accounted for, as the radiative lifetime (10 μs) is short compared to other time constants. The relevant rate coefficients are defined in Figure 2. A typical set of parameters used is given in the caption of Figure 5. All results are rather insensitive to the initial conditions. The relaxation of the rotational states has been included by choosing three rate coefficients for the exothermic 6→4, 4→2, and 2→0
Figure 4. Doppler profile of the R11(0.5) line, measured both with laser induced inhibition of complex growth (LIICG, decrease in the number of N+2 −He) and with laser induced charge transfer (LICT, increase in the number of Ar+). The linewidths indicate that the translational temperature of the ions can be as high as 57 K (LIICG) and 38 K (LICT).
this broadening is exclusively due to the Doppler effect implies a translational temperature, Ttrans = 46 K. This may be due to parasitic heating of the ions, a known effect in cold traps.15 In any case, the ions are rotationally cold because the internal temperature is the mass weighted average of the translational temperatures of the ions and the buffer gas (eq (6.2) in ref.16). For T trans = 46 K and T He = 5.5 K one obtains Trot = (4 Ttrans + 28 THe)/32 = 10.6 K. Several tests have been performed to prove that the measured transitions are due to absorptions of the bare N+2 ions and do not include overlapping transitions from laser induced fragmentation of N+2 −He. All observed lines are, within an accuracy of better than 0.005 cm−1, in agreement with the known spectroscopic constants of N+2 . Another observation is the recording of the R11(0.5) line by two different approaches (Figure 4). The upper trace is the signal detected by LIICG while the lower one via LICT. The latter method is described in ref 9 and, based on the endothermic electron transfer reaction: N+2 + Ar → N2 + Ar+, which is possible under the cold conditions of the trap only when N+2 ions are vibrationally excited. Mixing a trace of argon into the helium gas and exciting N+2 with a laser leads to the formation of Ar+. The line positions measured with the two methods agree within 0.001 cm−1. In a further test, the ion ensemble was irradiated with the laser in the second half of the trapping cycle (Figure 1). The result was a larger number of N2+−He complexes because there was no laser-induced inhibition and apparently no fragmentation. Attempts to find optical transitions of the N+2 −He complex were unsuccessful. This may be due to fast predissociation in the excited A2Πu electronic state.
Figure 5. Simulation of the relative population of a trapped ortho-N+2 ensemble (rotational states N = 0, 2, 4, 6), “hot” N+2 * and N+2 −He products formed via ternary association (see Figure 2). Initial conditions: P(6) = 0.5 and P(N+2 *) = 0.5, P(all other) = 0, [He] = 4.75 × 1015 cm−3, Trot= 10.6 K, k3 = 1 × 10−31 cm6s−1, kCID = 1 × 10−14 cm3s−1, krel = 1 × 10−14 cm3s−1. See text concerning rotational relaxation. Without laser (“laser off”), 4.5% of complexes are formed. Excitation of N+2 via the N = 2 level in the X2Σ+g state with a rate of 500 s−1 reduces the number of products to 1.1% (“laser on”), and the rotational population reaches a nonstatistical equilibrium already before 50 ms. 4053
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Its Applications; Winefordner, J. D., Ed.; John Wiley & Sons Inc.: Hoboken, NJ, 2005; and references therein. (3) Gerlich, D. In Inhomogeneous RF Fields: A Versatile Tool for the Study of Processes with Slow Ions; Ng, C. Y., Baer, M., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 1992; DOI: 10.1002/9780470141397.ch1. (4) Martner, C. C.; Pfaff, J.; Rosenbaum, N. H.; Ò Keefe, A.; Saykally, R. J. Radiative Lifetimes of Trapped Molecular Ions: HCl+ and HBr+. J. Chem. Phys. 1983, 78, 7073−7076. (5) Dzhonson, A.; Jochnowitz, E. B.; Maier, J. P. Electronic GasPhase Spectra of Larger Polyacetylene Cations. J. Phys. Chem. A 2007, 111, 1887−1890. (6) Boyarkin, O. V.; Mercier, S. R.; Kamariotis, A.; Rizzo, T. R. Electronic Spectroscopy of Cold, Protonated Tryptophan and Tyrosine. J. Am. Chem. Soc. 2006, 128, 2816−2817. (7) Jašík, J.; Ž abka, J.; Roithová, J.; Gerlich, D. Infrared Spectroscopy of Trapped Molecular Dications below 4 K. Int. J. Mass Spectrom. 2013, DOI: 10.1016/j.ijms.2013.06.007. (8) Schlemmer, S.; Asvany, O. Laser Induced Reactions in a 22-Pole Trap. J. Phys.: Conf. Series 2005, 4, 134−141. (9) Schlemmer, S.; Kuhn, T.; Lescop, E.; Gerlich, D. Laser Excited N+2 in a 22-Pole Trap, Experimental Studies of Rotational Relaxation Processes. Int. J. Mass Spectrom. 1999, 185, 589−602. (10) Asvany, O.; Ricken, O.; Müller, H. S. P.; Wiedner, M. C.; Giesen, T. F.; Schlemmer, S. High-Resolution Rotational Spectroscopy in a Cold Ion Trap: H2D+ and D2H+. Phys. Rev. Lett. 2008, 100, 233004. (11) Asvany, O.; Kumar, P, P.; Redlich, B.; Hegemann, I.; Schlemmer, S.; Marx, D. Understanding the Infrared Spectrum of Bare CH+5 . Science 2005, 309, 1219−1222. (12) Bieske, E. J.; Dopfer, O. High-Resolution Spectroscopy of Cluster Ions. Chem. Rev. 2000, 100, 3963−3998. (13) Zellweger, J. M.; Philippoz, J. M.; Melinon, P.; Monot, R.; van den Bergh, H. Isotopically Selective Condensation and Infrared-LaserAssisted Gas−Dynamic Isotope Separation. Phys. Rev. Lett. 1984, 52, 522−525. (14) Tanuma, H.; Fujimatsu, H.; Kobayashi, N. Ion Mobility Measurements and Thermal Transpiration Effects in Helium Gas at 4.3 K. J. Chem. Phys. 2000, 113, 1738−1744. (15) Asvany, O.; Schlemmer, S. Numerical Simulations of Kinetic Ion Temperature in a Cryogenic Linear Multipole Trap. Int. J. Mass Spectrom. 2009, 279, 147−155. (16) Gerlich, D. The Production and Study of Ultra-Cold Molecular Ions. In Low Temperatures and Cold Molecules; Smith, I., Ed.; Imperial College Press: London, 2008; Chapter 6, pp 295−343. (17) Stoecklin, T.; Voronin, A. Strong Isotope Eeffect in Ultracold Collision of N+2 (v = 1, j = 0) with He: A Case Study of Virtual-State Scattering. Phys. Rev. A 2005, 72, 042714.
transitions, while the ones in the opposite direction are determined by microreversibility. The data shown in Figure 5 have been obtained with kN→N−2 = 10−13 cm3 s−1. This is most likely too slow, because rotational states can change with almost the Langevin rate coefficient of 6 × 10−10 cm3 s−1, but with the high buffer gas density used in the present experiment, it is still much faster than the other processes occurring in the trap. Important is the vibrational relaxation of N+2 , requiring 105 collisions with helium. The determined rate coefficient of 10−14 cm3 s−1 is in accord with theoretical work.17 The rotational population is equilibrated rather early (Figure 5, laser off) while relaxation of “hot” N+2 , with an initial population of 50% (due to electron bombardment in the source), takes longer. The net production of N+2 −He complexes is inefficient because of collision induced fragmentation. Although the used rate coefficient is only 10−14 cm3s−1, it is responsible for the stationary equilibrium of 4.5%, reached after 100 ms. Nonetheless, these conditions are sufficient for recording a laser induced spectrum as can be concluded from Figure 5 (laser on). The number of N+2 −He is reduced to 1.1% if the N+2 ions are excited with a rate of 500 s−1. This result is in agreement with the observation of a low number of N+2 −He complexes but large laser induced changes. A more systematic variation of experimental parameters and simulations are necessary to determine the relevant rate coefficients. For example, the measured number of complexes increases with density, but not quadratically as expected from ternary association alone. Also a steep falloff in the number of N2+−He with increasing temperature has been observed, indicating that the trap should be operated at the lowest temperature possible. In summary, a novel method for measuring spectra of cold ions has been developed. The electronic spectrum of N+2 could be recorded in spite of a ternary rate coefficient of only 10−31 cm6s−1 for N+2 −He formation. The technique is universal, because complexation of ions with helium can be impeded over a wide wavelength range. Depending on the molecule and the detailed kinetics of interest, the experimental parameters have to be adjusted: laser power, trap temperature, gas density and storage time. Thus, up to 1010 collisions can be realized. In the case of larger ions, internal conversion following electronic excitation forms “hot” ions, reducing the attachment of helium and thus making the method applicable.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been financially supported by the European Research Council (ERC-AdG-ElecSpecIons: 246998) and the Swiss National Science Foundation (Project 200020-140316/1).
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REFERENCES
(1) Nagarajan, R.; Maier, J. P. Electronic Spectra of Carbon Chains and Derivatives. Int. Rev. Phys. Chem. 2010, 29, 521−554 and references therein.. (2) March, R.; Todd, J. F. In Quadrupole Ion Trap Mass Spectrometry, Chemical Analysis: A Series of Monographs on Analytical Chemistry and 4054
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