Chirped-Pulse in Uniform Flow

Transcription

1 Chirped-Pulse in Uniform Flow Baptiste Joalland Chamara Abeysekera, Lindsay N. Zack, Nuwandi M. Ariyasingha Ian R. Sims, Robert W. Field, Arthur G. Suits

2 Current Challenges for Laboratory Astrophysics How to Determine Reaction Rates and Low-Temperature Branching Ratios? Nearly all kinetics studies report the observed rate of reactant disappearance, with product identity and branching largely unknown. Recent advances have relied upon tunable VUV from synchrotron sources. However, these studies require complex fitting to infer branching, and clear product signatures are often lacking. [Trevitt et al. 211] Baptiste Joalland SF2A SF2A - Toulouse Toulouse

3 A New Approach [G. G. Brown et al, 28] Chirped-Pulse MicroWave Spectroscopy developed by Brooks Pate and migrated to higher frequencies by R.W. Field and coworkers combined with [I. R. Sims et al, 1994] Pulsed Uniform Supersonic Flows thermalized flows at ~2 K and high density via Laval nozzle, developed by B.R. Rowe, I.W.M. Smith, and I.R. Sims - CRESU Rotational spectroscopy becomes a tool for Kinetics/Dynamics and Astrochemistry Baptiste Joalland SF2A SF2A - Toulouse Toulouse

4 Chirped- Pulse Microwave Spectroscopy Apply RF B 1 field in xy-plane, all spins now precess with the same phase. Analogy with NMR Detect the spontaneous emission (Free Induction Decay) time Fourier transform to obtain the spectrum Apply RF B field Baptiste Joalland SF2A SF2A - Toulouse Toulouse

5 Chirped- Pulse Microwave Spectroscopy What is so special about it? GHz Same peak power CP cavity cavity chirped- pulse No need to step cavity dimension Pulse duration and power are independent Signal scales as (bandwidth) -1/2

6 Chirped- Pulse Microwave Spectroscopy What is so special about it? 17 min. 14 hrs. This suggests broadband rotational spectroscopy could be used, like REMPI or LIF or VUV, as a very general probe of reaction dynamics, with detailed structural information at the same time.

7 Chirped- Pulse Microwave Spectroscopy Chirped pulse generated by arbitrary waveform generator and upconverted to desired region Broadband excitation (several GHz) possible in a few ns Low frequency system (Ka band): 26-4 GHz peak power 4 W High frequency system (W band) 6-9 GHz peak power 1 mw Collaboration with Field MIT

8 Chirped- Pulse Microwave Spectroscopy How do we maximize signal? Linear sweep rate Popula:on difference between adjacent levels => CRESU Kinetics of Reaction in Supersonic Uniform Flows (CRESU) Laval nozzle c p T = c p T + ½ u 2 Vacuum chamber gas pulse Gas Pulse laser Laser CHIRPS FID + Chirp +FID => segmented chirps 1µs 2µs 1µs FFT FT Throat diameter 3 mm 5 cm Supersonic uniform flow T = K n = cm 3 Collaboration with I.R. IPR

9 8 Pulsed Uniform Supersonic Flow (Pulsed CRESU) 3 adapter (Al) 13 nozzle (SS 1mm) 2 nut 1 micrometer 6 teflon guide 7 plunger (SS or Al) 12 ferrule (Al) 11 chamber cap (SS) 3 mm Piezoelectric stack valve Pitot adjustment T rot = 2-3 K Reservoir Transducer Polycarbonate flow chamber Slip-gas needle valve Laval nozzle CP-FTMW receiving horn Pitot tube Turbomolecular pump CP-FTMW transmitting horn Translational stage laser [J.M. Oldham, C. Abeysekara, B. Joalland, L.N. Zack, K. Prozument, G.B. Park, I.R. Sims., R.W. Field, A.G. Suits, Journal of Chemical Physics, 214] 6 mm [C. Abeysekara, L.N. Zack, G.B. Park, B. Joalland, J.M. Oldham, K. Prozument, N.M. Ariyasingha, I.R. Sims., R.W. Field, A.G. Suits, Journal of Chemical Physics, 214]

10 Chirped- Pulse in Uniform Flow Pulsed Uniform Supersonic Flow (Pulsed CRESU) Ar Nozzle Excellent agreement between flow temperature and rotational temperature [C. Abeysekara, L.N. Zack, G.B. Park, B. Joalland, J.M. Oldham, K. Prozument, N.M. Ariyasingha, I.R. Sims., R.W. Field, A.G. Suits, Journal of Chemical Physics, 214]

11 Chirped- Pulse in Uniform Flow On the (High) Collisional Environment dimethyl-ether CH3OCH3 mean free path ~ 3 μm ; mean time 9 ns up down up down => the coherence time is shortened by dephasing phenomena

12 Chirped- Pulse in Uniform Flows Applications for Reaction Dynamics 193 nm Photodissociation of SO2 at 193nm: Probing the nascent vibrational SO (X 3 Σ - ) Reaction of CN+C2H2 at 2K: Probing HC3N 1 Vibrational population % v = v =2 v = Time (µs) Population (%) v REMPI + VMI a LIF b IR c CPUF d <1... a [C. Abeysekara, L.N. Zack, G.B. Park, B. Joalland, J.M. Oldham, K. Prozument, N.M. Ariyasingha, I.R. Sims., R.W. Field, A.G. Suits, Journal of Chemical Physics, 214]

13 Chirped- Pulse in Uniform Flow Applica:ons for Mul:channel Reac:ons CN + propyne 22K CBS-QB3 level of theory

14 analysis above assumes temperature. That this 6 CN + CH 3 CCH itial expansion into the 5 HC 3 N J = 9 8 Intensity (μv) HC3N J = 9-8 ional spectra for the he- 4 s. We have not been able 3 d there are several pos- 2 argon is a much greater 1 chaperone effect ins a much lower Frequency (GHz) thermal 6 at accompanies CN + CH 3 CCH the inicase our impact CH 3C 3N pres- 5 J = y overestimate the flow 3 2 ure experiments with a 1 stigate this. Intensity (μv) are closed-shell asymmetric tops containing either one o methyl rotors, which cause each J (K a, K c ) J (K a, rotational transition to split into multiple components acetaldehyde, the methyl rotation results in fully-resol doublets, denoted A and E. Thus, the analysis for ace hyde was based on 12 distinct spectral lines. Dimethyl Chirped- Pulse in Uniform Flow Applica:ons for Mul:channel Reac:ons CN + propyne 22K however, 25 contains two methyl rotors, which produce qu CH3C3N J = 2-19 CH 3 C 3 N J = HCN were recorded, lines belonging to the samej = J = 1 J - (K 1 CH3C3N 3 C 3 N a, J = 21-2 V 5 Intensity / µv CH 3 CCH + CN of blended or partially-blended lines (AA, EE, AE, and 2 Therefore, although 15 spectral features due to CH 3 O J (K a, K c )transitionwerecollapsedandthecente quency taken as the average frequency of the quartet an total 82.6 line intensity as 87.2 the87.4sum87.6 of87.8 the 88. individual intensitie 88.6 Frequency (GHz) The Boltzmann plots were constructed using the fo ing relationship between the integrated line intensities and corresponding lower Integrated stateline energies intensity: (E l ): 36 W = 4π 3/2 ω 2 Sµ2 i g ı g I ε c α N tot kt Q rot e E ı /kt rot, Frequency (GHz)

15 Chirped- Pulse in Uniform Flow Applica:ons for Mul:channel Reac:ons CN + propyne 22K branching (%): CPUF C1 RRKM C2 12(5) (4) (6) (8) 7 4 [C. Abeysekara, B. Joalland, N.M. Ariyasingha, L.N. Zack, I.R. Sims., R.W. Field, A.G. Suits, Journal of Physical Chemistry Letters, 215]

16 The Future of C- PUF Direct Digital Synthesis Replacing AWG at low cost Blake Group in Caltech Frequency (MHz) 2 1 AWG.5 1 Time (µs) Frequency (MHz) 2 1 DDS.5 1 Time (µs) Power (dbc) Electric Field (Volts) Time (µs) Electric Field (Volts) Time (µs) Frequency Direct Digital Magnitude (Arb. Units) Frequency (MHz) [I. A. Finneran, D. B. Holland, P. B. Carroll, and G. A. Blake, Review of Scientific Instruments, 213] Magnitude (Arb. Units) Frequency (MHz)

17 The Future of C- PUF From GHz to THz: A Strong Overlap with Radioastronomy Observing facilities reach sensitivities to probe beyond simple molecules and on spatial resolution to reveal star and planet formation in action e.g. ALMA, NOEMA ALMA: GHz, up to ~1km baselines spatial resolution <.1 at low frequencies sensitivity to detect more complex low abundance species in star forming regions and disks spatial distribution and kinematics of molecules within cores, embedded protostars and disks CO! => pathways to molecular complexity [Mannings!et!al.!1997]! methyl cyanide in a protoplanetary disk [Oeberg!et!al.!215]!

18 Acknowledgments Prof. Arthur G. Suits Collaborators: Prof. Robert W. Field (MIT) Dr. G. Barratt Park (MIT) Prof. Ian Sims (IPR/Rennes) Suits Group: Chamara Abeysekera Lindsay Zack Nuwandi Ariyasingha James Oldham Kirill Prozument Fundings: