Photo-Dissociation Resonances of Jet-Cooled NO 2 by CW-CRDS

Transcription

1 Photo-Dissociation Resonances of Jet-Cooled NO 2 by CW-CRDS Patrick DUPRÉ Laboratoire de Physico-Chimie de l Atmosphère, Université du Littoral, Côte d Opale Dunkerque, France ISMS June 2015 Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

2 Outline 1 Introduction 2 RRKM Microcanonical Predictions 3 Experimental Setup 4 Jet-Cooled NO 2 at the Dissociation Threshold by CRDS 5 Spectroscopy Background 6 Analysis of the Transitions 7 Analysis of the Resonances 8 Summary/Conclusions Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

3 Motivations Molecular Dissociation remains Controverted; nevertheless, there are Fascinating Chemical Physics Processes E.g. Roaming observed in H 2 CO, NO 3, CH 3 CHO (photodissociation/bimodal vibrational distribution) Location of the Transition States (TS) along a barrierless reaction path? Benchmark the different variants of the RRKM (Rice Ramsperger Kassel Marcus) formalisms Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

4 About NO 2 NO 2 : a simple molecule exhibiting a conical interaction above cm 1 The jet-cooled LIF spectrum of NO 2 exhibits a well identified dissociation threshold, NO 2 NO ( X 2 Π 1/2, v = 0 ) + O( 3 P 2 ): Abrupt Disappearance of the Fluorescence around 398 nm, PHOFEX (Jet-Cooled) Photofragments up to D cm 1 Unimolecular Dissociation Rate up to D cm 1 Photodissociation Resonances up to D cm 1 Character of the TS: Loose or Tight? Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

5 Comparison with other Experiments New Spectrum CW-CRDS (Absorption-based) Obtained under CW Jet-Cooled Slit Expansion Nanosecond Experiments Pulsed Laser Source Pulsed Jet Expansion (Pin-Hole) PHOFEX (NO or O?), Depletion, TC-LIGS Spectral or Time resolved Data Comparison Residual Doppler Broadening improved by a factor 5 ( 130MHz, HWHM) Pure Absorption (no fragment analysis) Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

6 RRKM Microcanonical Predictions Statistical Reaction Rate Coefficient (assumption: energy conservation): ( ( k uni E ) = N E ) h ρ (E ) N ( E ) : Number of Levels at the TS For Barrierless Reaction, the coordinate of the TS (R ) should match a large Interfragment Distance (loose states), ρ ( E ) : Density of Reactant Levels, Variant RRKM Theories: Variational RRKM: Energy and Angular Momentum Conservation, SACM (Statistical Adiabatic Channel Model), Phase Space Theory (PST), etc... Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

7 Experimental Setup Patrick DUPRÉ (LPCA/ULCO) Resonances in NO2 at D0 ISMS / 28

8 LIF versus CRDS of Jet-Cooled NO 2 around D 0 Absorbance (ppm/pass.) τ e ~ 30 µs Slit: 45 µm x 7 mm Seeding: 0.01 % of NO 2 in He at 1.5 bar Excitation Energy (cm 1 ) Dissociation Threshold Time Integrated LIF (a.u.)

9 Spectroscopy Background In J-scheme of coupling: F = J +I, J = N +S (near symmetric top) H = H rot + H sr + H FC + H SI J N F 1 F F 3/2 F 1/2 F 1/2 F + 1/2 F + 1 F + 1/2 F + 3/2 with: F 3/2 H SI : N = 0, ±1, ±2; J = 0, ±1; K = 0, ±2 H FC : N = 0; J = 0, ±1; K = 0 F 1/2 F = I = S = 0 N K H N K = ( 1) K N K H N K H rot = AN 2 a +B N2 b +C N2 c F 1/2 Modified WangTransformation F + 1/2 Diagonalization = a ± S I F τ k H sr = 1 (N ɛs +S ɛn) 2 H FC = a FC I S H SI = g S g I µ B µ N [ I S r 3 afc + T (2) 0 (r ) T (2) 0 (r ) T (2) ±2 (r ) T (2) ( ±2 (r ) ) T (0) 0 (r ) = 0 FermiContact+Spin-Electronic Spin-Nuclear DipoleInteraction (Orthorhombic) 3(I ] r)(s r) r 5 = I T(r) S I = 1 S = 1/2

10 Fine/Hyperfine Coupling: Pairs [ q R 0 (0), q P 2 (0)] N = 1 J = 3/2 + J = 1/2 2 B 2 F 5/2 3/2 1/2 1/2 3/2 R0 P2 Q21(1/2) R11(1/2) P11(5/2) Q12(3/2) P22(3/2) N = 2 N = 0 2 A cm 1 J = 3/2 + J = 5/2 + J = 1/ cm cm 1 1/2 3/2 5/2 7/2 5/2 3/2 3/2 1/2 F 221 MHz

11 Analysis of the Transitions CW-CRDS of Jet-Cooled NO 2 at D Absorption (/cm) Dissociation Threshold Excitation Energy (cm 1 )

12 Summary of the Transition Analysis Comparison with previous LIF experiments (Jet-Cooled/Pinhole, Observation Slit): Temperature slightly higher, Experimental resolution slightly worst Full Simulation of the Transition (STEPRAM) Transition Shape: q R 0 (0) versus q P 0 (2), etc... In the range D cm 1 D 0 : only q R 0 (0) transitions are observed 49 pairs identified between and cm 1 Hyperfine Structures in the Upper Level are weak (the Fermi-Contact term in the state à is negligible) Upper Level Fine Structure? Two times more levels J = 3/2 than J = 1/2 (1:2 ratio amplitude) Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

13 Analysis of the Resonances Absorption (/cm) Γ = (10) cm Excitation Wavenumber (cm 1 )

14 Resonance Analysis Absorption (/cm) Excitation Energy (cm 1 )

15 Resonance Width Analysis Linewidth (HWHM) (cm 1 ) Dissociation Time (ps) Excitation Wavenumber (cm 1 )

16 Spectral Analysis Hyperfine Structure ignored (J assumed) Resonances and Transitions can be distinguished without ambiguity A few ( hot ) Resonances below D 0 : One clearly Isolated ( 50ps) Around 115 individual Resonances fitted Large Resonance Width Fluctuations (2 orders of magnitude) Resonance Overlapping: from weak to strong Continuum Background? Pairing of only a few Resonances Stepwise at D cm 1 i.e., D 0 +3B NO (J NO = 1/2 to J NO = 3/2), in agreement with PST (channel at D 0 +8B NO?) Qualitative Comparison with ab initio calculations/sacm (scattering matrix) Comparison with PHOFEX Data: 2 times more Resonances, and Narrower Resonances are observed Comparison with Time Resolved Experiments: a lack of spectral resolution

17 Uni-molecular Dissociation Rates Weighting: Uniform Standard error Resonance Strength k uni σ kuni k uni σ kuni k uni σ kuni n dof E < D cm D cm 1 E < D cm Values in ps 1 k uni1/ ps 1 ( 61 ps)comparedwithk uni3/ ps 1 ( 18 ps), 3 overlapping regimes (ρ res Γ 1)? Feshbach Theory of Resonance scattering, Analysis of σ kuni : Number of Degrees of Freedom (n dof from 12.5 to 8.3), Comparison with PHOFEX (spectral resolution/temperature): Broader Resonance Width Distribution, and Density of Resonances 3 Times Larger, but very close value of k uni Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

18 Densities at Dissociation ρ reac 11.2 levels/cm 1 (weighting according to the relative number of J values) ρ res 9.5/cm 1 (PHOFEX: ρ peak 3.4/cm 1 ) LIF: ρ trans = 16.6/cm 1 : ρ vib = 5.8 ±0.24 levels/cm 1 has been deduced ρ res (9.5/cm 1 ) ρ trans (16.6/cm 1 ): absence of failure in the density of vibronic levels at D 0? ρ vib = 5.8 ±0.24 levels/cm 1 : high compared with the extrapolation by Dunham expansions < 1 levels/cm 1 Resonances versus Roaming States? Interpretation: Floppy Frame of the Molecule associated with Large Bond? Scrambling? Rovibronic coupling? In agreement with the Loose TS? Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28

19 Summary/Conclusions The frequency Range D cm 1 D 0 Transition versus Resonance Pairing (R 0, P 2 ) Hyperfine Structures (Upper versus Lower Levels) Large Quantal Dispersion of the Resonance Width (1:100) Dissociation Channel Openings (at least 2 are observed) Compatibility with Phase Space Theory: k uni1/ ps 1 ( 61 ps) compared with k uni3/ ps 1 ( 18 ps) Loosely Bound TS Roaming? Reconciling Spectral and Time Resolved Experiments: from Loose to Tight TS? The High Resolution of the CW-CRDS only helps below D 0 +15cm 1 Perspectives Cooler Radicals Analysis of the Product Distribution (CW) Ab initio Calculations (PESs) Isotopologues Cold Femtosecond Pump-Probe Experiments?

20 Thanks Thank for your Attention Ref: J. Chem. Phys. 142, (2015) Patrick DUPRÉ (LPCA/ULCO) Resonances in NO 2 at D 0 ISMS / 28