Suppression of Spontaneous Emission in the Optical Pumping of

Letter pubs.acs.org/JPCL

Suppression of Spontaneous Emission in the Optical Pumping of Molecules: Pump−Dump−Sweep−Probe Nils Bartels,† Bastian C. Krüger,† Sven Meyer,† Alec M. Wodtke,†,‡ and Tim Schaf̈ er*,†,‡ †

Institut für Physikalische Chemie, Georg-August University of Göttingen, Tammannstraße 6, 37077 Göttingen, Germany Department of Dynamics at Surfaces, Max Planck Institut für biophysikalische Chemie, Am Faßberg 11, 37077 Göttingen, Germany

‡

ABSTRACT: Optical pumping experiments on molecules are often complicated by spontaneous emission, which competes with stimulated absorption and emission processes. One well-known example is stimulated emission pumping (SEP), an optical pumping method relying on a Λ-transition via an excited electronic state to produce highly vibrationally excited molecules in their ground electronic state. Here, spontaneous emission populates a host of untargeted vibrational states in the optical pumping. In this Letter, we report a novel approach to suppressing spontaneous emission and show its application by preparing NO molecules in X2Π(ν = 16) via the transient A2Σ+(ν = 2) state. Subsequent to nanosecond time scale optical pumping, we depopulate the upper state in the Λ-transition by selective resonant excitation to rapidly dissociating states. We demonstrate reduction in the spontaneous emission from the A2Σ+(ν = 2) state by more than 90%. Because this method employs pulsed lasers, it is easily implemented in the ultraviolet. SECTION: Spectroscopy, Photochemistry, and Excited States excited O2 in stratospheric ozone formation,5−7 resonant ν−ν energy transfer,8 the infrared radiative lifetimes of highly vibrationally excited molecules,9 vibrational promotion of electron transfer,10 and the breakdown of the Born− Oppenheimer approximation in molecule surface interactions,11 to name just a few. Spontaneous emission from the excited electronic state used in the Λ-transition (in the case shown in Figure 1, the A2Σ+(ν = 2) state of NO) is a collision-free process that can produce population in the same states detected by the probe laser. Figure 2 (left column) shows a pump−pump−probe experiment where NO X2Π(ν = 16) is produced by SEP and allowed to collide at the (111) surface of a clean Au crystal. The probe laser is used to detect population appearing in NO X2Π(ν = 6) (upper left panel). Naively, one would expect the signal seen here to reflect the probability for a collision-induced transition, NO X2Π(ν = 16 → 6). However, the lower panel shows the same spectrum when the dump laser is blocked, eliminating the population of NO in X2Π(ν = 16). Here, spontaneous emission from A2Σ+(ν = 2) to a variety of vibrational states is followed by collision with the Au(111) surface populating NO X2Π(ν = 6) . This is an example of an optical pumping experiment that is completely ruined by spontaneous emission. Figure 2 (right column) shows the results of a similar experiment when a sweep laser pulse is used to suppress spontaneous emission. In this experiment, a 5 ns laser pulse is used to transfer population out of A2Σ+(ν = 2) to a high-lying excited electronic state that rapidly predissociates. The sweep

L

aser-based optical pumping has long been an important tool for studying the properties of the excited electronic states of molecules.1 Methods to study highly vibrationally excited quantum levels of molecules in their ground electronic state have only more recently become available.2 This represents a particularly interesting challenge for optical pumping as energized ground electronic state molecules play an important role in chemistry but can hardly be populated by direct overtone excitation from their vibrational ground state as the oscillator strengths for overtone excitation fall off rapidly with increasing vibrational target state. Stimulated emission pumping (SEP) is a remarkably powerful approach capable of meeting this challenge.3,4 Here, a Λ-transition via an excited electronic state is employed. By using an intermediate electronic state with favorable Franck− Condon factors, one can readily prepare molecules in individual quantum states with several electronvolts of vibrational energy. As an example, Figure 1 shows how population can be produced in the 16th excited vibrational state of electronic ground state NO X2Π(ν = 16). A PUMP laser excites a rotationally resolved transition in the A2Σ+(ν = 2) ← X2Π(ν = 0) band. Subsequently, a dump laser stimulates emission, transferring population to the target state via the A2Σ+(ν = 2) → X2Π(ν = 16) band. This so-called pump−dump technique is capable of producing large enough populations of highly vibrationally excited molecules for scattering experiments (pump−dump− probe). Here, a probe laser pulse fires several microseconds later to interrogate the population distribution induced by an intervening collisional event. The pump−dump−probe method has been applied to several problems in molecular physics and physical chemistry including the role of highly vibrationally © 2013 American Chemical Society

Received: June 19, 2013 Accepted: July 1, 2013 Published: July 1, 2013 2367

dx.doi.org/10.1021/jz401266m | J. Phys. Chem. Lett. 2013, 4, 2367−2370

The Journal of Physical Chemistry Letters

Letter

comparing fluorescence decay traces like this, we can determine the degree to which spontaneous emission is suppressed (>90%). The insets of Figure 3 show the population distributions produced in these three experiments. For conventional SEP (middle panel), the population of NO molecules in ν = 6−13, which is produced by spontaneous emission, is comparable to the population of ν = 16 produced by stimulated emission. Molecules in these states clearly scatter to ν = 6 by processes like NO X2Π(ν = 6 → 6,ν = 7 → 6,ν = 8 → 6), and so forth. This proceeds efficiently enough to overwhelm the scattering channel of interest, namely, the NO X2Π(ν = 16 → 6). By contrast, the sweep pulse produces a remarkable reduction in population of states produced by spontaneous emission, revealing the NO X2Π(ν = 16 → 6) channel. Pump−dump−sweep is not the first approach to removing the influence of spontaneous emission in optical pumping experiments. An alternative method is stimulated Raman adiabatic passage (STIRAP), which is a highly successful method for overcoming the influence of spontaneous emission in optical pumping in molecules16 and has been crucial in the production of bound cold molecules in photoassociation experiments.17 Unfortunately, its application requires highly coherent light sources and is most straightforward when singlemode cw lasers are available. STIRAP also scales unfavorably with shorter pulse duration, even when the pulse is coherent; STIRAP with coherent laser pulses shorter than about 5 ns has not been demonstrated. While nanosecond-pulsed coherent lasers are available and indeed pulsed STIRAP has been demonstrated,18−20 its implementation can be challenging. Indeed, STIRAP on NO using similar transitions to those in this work resulted in less suppression of spontaneous emission than we have demonstrated with the pump−dump−sweep approach.21 In contrast to STIRAP, the pump−dump−sweep approach offers advantages of simplicity. Most importantly, it is a concept based on incoherent pumping and therefore does not require Fourier transform limited light pulses. In summary, we have demonstrated an alternative approach to suppressing spontaneous emission in optical pumping experiments that is based on multimode nanosecond-pulsed laser pulses that operate efficiently to produce UV light. The intermediate states used in a nanosecond-pulsed Λ-transition are resonantly photodissociated by a pulsed nanosecond laser. This simple idea dramatically improves the quantum state purity of the optically prepared sample.

Figure 1. Schematic diagram of the optical pumping scheme. Stimulated emission pumping (SEP) of NO proceeds via a twocolor Λ-transition; population is transferred from X2Π(ν = 0) to X2Π(ν = 16) via the A2Σ+(ν = 2). The 5 ns pump laser pulse, λ1, is followed by another 5 ns dump laser pulse, λ2. Spontaneous emission from A2Σ+(ν = 2) is suppressed by state-selective photodissociation with a sweep laser pulse, λ3, that directly follows the dump laser pulse. Population transfer is detected by a probe pulse, λ4, after the sample collides with a crystalline gold sample, transferring population to lower vibrational states.

pulse is fired immediately after the pump and dump pulses. The impact of the sweep pulse is dramatic. Now, the probe laser signal detecting NO X2Π(ν = 6) is strongly enhanced by the dump laser pulse, that is, by the optically prepared population of NO in X2Π(ν = 16). The background produced by spontaneous emission is significantly smaller than the desired pump−dump−probe signal allowing data to be obtained, reflecting the probability of the collision-induced transition, NO X2Π(ν = 16 → 6). Figure 3 shows in more detail the nature of the pump− dump−sweep−probe experiment. In the main panels, we show the spontaneous emission transient observed on a photomultiplier. The left panel shows the spontaneous emission decay induced by the pump laser pulse. The characteristic exponential decay time is consistent with previous work.12−15 The middle panel shows spontaneous emission occurring with both pump and dump laser pulses firing. Here, the dump laser pulse, which is overlapped with the pump laser pulse in space and time, depletes the spontaneous emission by 20%. This fluorescence depletion is typical of SEP (the maximum depletion that can theoretically be achieved is 33%). The right panel shows the spontaneous emission detected on the photomultiplier during the pump−dump−sweep−probe experiment. Here, the sweep laser is fired immediately after the pump and dump event to optimally suppress spontaneous emission without reducing population transfer to ν = 16. By

■

EXPERIMENTAL METHODS The experiments are carried out in a molecular beam apparatus similar to that described in previous papers.22,23 Rotationally cold NO molecules (TROT ≈ 6 K) are produced by expanding a mixture of 5% NO in H2 (Ekin = 0. 68 eV) into the vacuum through a piezoelectric valve (1 mm Ø nozzle, 10 Hz, 3 atm stagnation pressure). The population of NO X2Π1/2(ν″ = 16, J = 0.5) is produced by exciting the A2Σ+(ν′ = 2) ← X2Π1/2(ν″ = 0) Q11(0.5) (pump) transition at 204.708 nm (λ1) followed by de-excitation via the A2Σ+(ν′ = 2) → X2Π1/2(ν″ = 16) Q11(0.5) (dump) transition at 450.503 nm (λ2). These pulses are generated using two narrow-bandwidth home-built OPO laser systems,24 which are pumped by the second harmonic of the same Nd:YAG laser (LAB-170-10, Spectra Physics). The IR outputs of the OPOs are then either mixed with the fourth harmonic of the Nd:YAG laser or frequency doubled in BBO 2368



Letter

Figure 2. Effect of pump−dump−sweep probe on a scattering experiment. A pump−dump−probe experiment is performed where NO X2Π(ν = 16) is produced by SEP and allowed to collide at the (111) surface of a clean Au crystal. The probe laser is used to detect population appearing in NO X2Π(ν = 6) via the A2Σ+(ν = 2) ← X2Π(ν = 6) member of the γ-bands by 1 + 1 REMPI. For both panels of the left column, the sweep pulse is not used. The influence of the dump laser, which produces NO X2Π(ν = 16), is difficult to discern. The detected REMPI signal is dominated by scattering events involving vibrationally excited NO produced by spontaneous emission that appears in X2Π(ν = 6) after collision with the surface. The right column panels show the effect of suppressing spontaneous emission by implementing the sweep laser. The influence of the dump laser is now clearly seen.

Figure 3. Fluorescence transients observed on a photomultiplier. (Left) Spontaneous emission from NO A2Σ+(ν = 2) induced by the pump laser. (Middle) Fluorescence transient observed in a conventional SEP experiment. The dump laser induces a depletion of the fluorescence by 20%. (Right) The spontaneous emission signal observed in a pump−dump−sweep configuration. Here, the sweep laser removes population from A2Σ+(ν = 2) immediately after the pump and dump event. Insets show the calculated population distributions in each case.

the tested wavelength range between 445 and 455 nm. Even though we did not assign these transitions, exciting to appropriate predissociative states is undemanding due to the large number of electronically excited doublet states of NO above 6.5 eV.25 Fluorescence in the region of optical pumping (see Figure 3) is monitored using a quartz lens and a photomultiplier tube (Hamamatsu, R7154).

crystals to obtain the pump and dump pulses with typical pulse energies of 0.3 and 0.5 mJ, respectively. To deplete the fluorescence from the A2Σ+(ν′ = 2) state, we use the fundamental of a Nd:YAG-pumped (PRO-270, Spectra Physics) dye laser (PRSC-DA-24, Sirah, 8 mJ pulse energy). A suitable transition at λ3 = 450.87 nm was easily found by performing fluorescence dip spectroscopy. We found five transitions with fluorescence depletions greater than 70% in 2369



Letter

(15) Luque, J.; Crosley, D. R. Transition Probabilities and Electronic Transition Moments of the A 2Σ+−X 2Π and D 2Σ+−X 2Π Systems of Nitric Oxide. J. Chem. Phys. 1999, 111, 7405−7415. (16) Bergmann, K.; Theuer, H.; Shore, B. W. Coherent Population Transfer among Quantum States of Atoms and Molecules. Rev. Mod. Phys. 1998, 70, 1003−1025. (17) Stuhl, B. K.; Sawyer, B. C.; Wang, D.; Ye, J. Magneto-Optical Trap for Polar Molecules. Phys. Rev. Lett. 2008, 101, 243002. (18) Schiemann, S.; Kuhn, A.; Steuerwald, S.; Bergmann, K. Efficient Coherent Population Transfer in NO Molecules Using Pulsed Lasers. Phys. Rev. Lett. 1993, 71, 3637−3640. (19) Kuhn, A.; Coulston, G. W.; He, G. Z.; Schiemann, S.; Bergmann, K.; Warren, W. S. Population Transfer by Stimulated Raman-Scattering with Delayed Pulses Using Spectrally Broad Light. J. Chem. Phys. 1992, 96, 4215−4223. (20) Halfmann, T.; Bergmann, K. Coherent Population Transfer and Dark Resonances in SO2. J. Chem. Phys. 1996, 104, 7068−7072. (21) Kuhn, A.; Steuerwald, S.; Bergmann, K. Coherent Population Transfer in NO with Pulsed Lasers: The Consequences of Hyperfine Structure, Doppler Broadening and Electromagnetically Induced Absorption. Eur. Phys. J. D 1998, 1, 57−70. (22) Chen, J.; Matsiev, D.; White, J. D.; Murphy, M.; Wodtke, A. M. Hexapole Transport and Focusing of Vibrationally Excited NO Molecules Prepared by Optical Pumping. Chem. Phys. 2004, 301, 161−172. (23) Matsiev, D.; Chen, J.; Murphy, M.; Wodtke, A. M. Transport and Focusing of Highly Vibrationally Excited NO Molecules. J. Chem. Phys. 2003, 118, 9477−9480. (24) Velarde, L.; Engelhart, D. P.; Matsiev, D.; LaRue, J.; Auerbach, D. J.; Wodtke, A. M. Generation of Tunable Narrow Bandwidth Nanosecond Pulses in the Deep Ultraviolet for Efficient Optical Pumping and High Resolution Spectroscopy. Rev. Sci. Instrum. 2010, 81, 063106. (25) Gilmore, F. R. Potential Energy Curves for N2, NO, O2 and Corresponding Ions. J. Quant. Spectrosc. Radiat. Transfer 1965, 5, 369− 390.

For REMPI detection (see Figure 2), surface scattered molecules are resonantly ionized with the output of an OPO laser system (Sunlite Ex with FX1 unit, Continuum) via the A2Σ+(ν = 0) state. The produced ions are then detected with a microchannel plate assembly (MCP- 050, Tectra, two plates in chevron configuration).

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.M.W. acknowledges support from the Alexander von Humboldt foundation.

■

REFERENCES

(1) Parmenter, C. S. Vibrational-Energy Flow within Excited Electronic States of Large Molecules. J. Phys. Chem. 1982, 86, 1735−1750. (2) Kittrell, C.; Abramson, E.; Kinsey, J. L.; McDonald, S. A.; Reisner, D. E.; Field, R. W.; Katayama, D. H. Selective Vibrational-Excitation by Stimulated Emission Pumping. J. Chem. Phys. 1981, 75, 2056−2059. (3) Silva, M.; Jongma, R.; Field, R. W.; Wodtke, A. M. The Dynamics of “Stretched Molecules”: Experimental Studies of Highly Vibrationally Excited Molecules with Stimulated Emission Pumping. Annu. Rev. Phys. Chem. 2001, 52, 811−852. (4) Drabbels, M.; Wodtke, A. M. Collisions and Chemistry of SuperExcited Molecules: Experiments Using the Pump−Dump−Probe Technique. J. Phys. Chem. A 1999, 103, 7142−7154. (5) Miller, R. L.; Suits, A. G.; Houston, P. L.; Toumi, R.; Mack, J. A.; Wodtke, A. M. The “Ozone Deficit” Problem: O2(X, v ≥ 26) + O(3P) from 226-nm Ozone Photodissociation. Science 1994, 265, 1831− 1838. (6) Price, J. M.; Mack, J. A.; Rogaski, C. A.; Wodtke, A. M. Vibrational-State-Specific Self-Relaxation Rate-Constant  Measurement of Highly Vibrationally Excited O2 (ν = 19−28). Chem. Phys. 1993, 175, 83−98. (7) Rogaski, C. A.; Price, J. M.; Mack, J. A.; Wodtke, A. M. Laboratory Evidence for a Possible Non-Ldt Mechanism of Stratospheric Ozone Formation. Geophys. Res. Lett. 1993, 20, 2885−2888. (8) Mack, J. A.; Mikulecky, K.; Wodtke, A. M. Resonant Vibration− Vibration Energy Transfer between Highly Vibrationally Excited O2 (X 3 − Σ g, ν = 15−26) and CO2, N2O, N2, and O3. J. Chem. Phys. 1996, 105, 4105−4116. (9) Drabbels, M.; Wodtke, A. M. The Determination of the Infrared Radiative Lifetimes of a Vibrationally Excited Neutral Molecule Using Stimulated-Emission-Pumping, Molecular-Beam Time-of-Flight. J. Chem. Phys. 1997, 106, 3024−3028. (10) Huang, Y. H.; Rettner, C. T.; Auerbach, D. J.; Wodtke, A. M. Vibrational Promotion of Electron Transfer. Science 2000, 290, 111− 114. (11) White, J. D.; Chen, J.; Matsiev, D.; Auerbach, D. J.; Wodtke, A. M. Conversion of Large-Amplitude Vibration to Electron Excitation at a Metal Surface. Nature 2005, 433, 503−505. (12) McDermid, I. S.; Laudenslager, J. B. Radiative Lifetimes and Electronic Quenching Rate Constants for Single-Photon-Excited Rotational Levels of NO (A 2Σ+, ν′ = 0). J. Quant. Spectrosc. Radiat. Transfer 1982, 27, 483−492. (13) Settersten, T. B.; Patterson, B. D.; Humphries, W. H. Radiative Lifetimes of NO A 2Σ+ (ν′ = 0,1,2) and the Electronic Transition Moment of the A 2Σ+−X 2Π System. J. Chem. Phys. 2009, 131, 104309. (14) Luque, J.; Crosley, D. R. Radiative and Predissociative Rates for NO A 2Σ+ ν′=0−5 and D 2Σ+ ν′=0−3. J. Chem. Phys. 2000, 112, 9411−9416. 2370


Suppression of Spontaneous Emission in the Optical Pumping of

Recommend Documents