65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 1 TIME-RESOLVED ROTATIONAL

65th OSU International Symposium on Molecular SpectroscopyJune 21-25, 2010

José Luis DoménechInstituto de Estructura de la Materia

1

TIME-RESOLVED ROTATIONAL ENERGY

TRANSFER AND SPECTRAL LINE

BROADENING IN ACETYLENE.

A HIGH RESOLUTION RAMAN STUDY.

J. L. DOMENECH, R. Z. MARTINEZ AND D. BERMEJO

Laser Spectroscopy Group, Molecular Physics Department

Instituto de Estructura de la Materia (CSIC)

Serrano 123, 28006 Madrid, Spain



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MOTIVATION

• Rotational energy transfer rates are relevant in a number of important problems: energy balance in astrophysical environments, non LTE problems, laser dynamics, formation of spectral line shapes, etc. (Dynamicist’s point of view)

• High accuracy PES’s combined some calculation (CC, CS, classical trajectories…) allow the calculation of both the state-to-state rotational energy transfer cross-sections and the pressure broadening coefficients in ample ranges of pressure and temperature. (Theoretician’s point of view)

• The knowledge of pressure broadening and line mixing coefficients is essential for the accurate retrieval of concentrations, temperature distributions, etc. in remote sensing applications (earth’s atmosphere, combustion diagnostics, astrophysical media). It provides insight into molecular collision dynamics and allows for a very stringent test of intermolecular PES’s. (Spectroscopist’s point of view)



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Introduction• Pressure broadening and line-mixing of vibration-rotation lines are largely

dominated by rotationally inelastic collisions.

• The collisional width of isotropic Raman Q-branch lines is not affected by reorienting elastic collisions or pure dephasing collisions, only by inelastic processes. If vibrational relaxation and vibrational dephasing processes are slow

• In principle, by an inversion procedure, the k’s can be obtained from the widths as a function of J. However, the number of unknowns is much larger than the experimental data available, and it is necessary to resort to fitting and scaling laws that relate the rate coefficients to a smaller number of parameters (EGL, IOS, ECS,…)

, , ( )(v ) (v ) ; with rate constant f i TX J i M X J f M E k

v v ''

'

(v, v ', ')

( )

12 2

2

elastic

f i

f i

j i j i

k

j i j i

J i J i

Q J i

P k kc

Pc

Isotropic Raman (polarized part of the Q-branches)

Dipole transitions and anisotropic Raman (O and S branches)



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Introduction• A more direct approach to obtain the state-to-state rates is to monitor

the time evolution of non-equilibrium populations with rotational state resolution. Time-resolved pump-probe techniques have been widely used to address this type of problems:– Pump: IR absorption, Stimulated Raman effect, etc.– Probe: LIF, REMPI, IR absorption, etc.

• If there are no other gain (pumping is over), or loss mechanisms (vibrational relaxation, fluorescence, diffusion out of the probe volume), the change of population of a given level is due to rotationally inelastic collisions, and can be expressed:

d ( ) ( ) d

( ) d

f f i ii f

i f fi f

f f i fi f

n t k n t P t

k n t P t

k k



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MOTIVATION

• Our group has some experience in the measurement of collisional effects on high resolution Raman and IR spectra.

• We have also developed a Raman-Raman double resonance technique for the study of vibrationaly excited states, that we have improved with time resolution capabilities and J selectivity.

• We have the tools to obtain both the state-to-state RET rate coefficients and the Raman pressure broadening coefficients with the same experimental setup.

• Can our experiments provide some information on: the interplay of elastic/inelastic processes, vibrational contributions to the line broadening, independence of the RET rates on the vibrational state, etc.?



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Raman-Raman double resonance with time resolution

Groundstate

J

J

J

591 nm

3210

3210

3210

Bombeo Espectroscopía

V=2

V=1

532 nm

Spectroscopy

Groundstate

J

J

J

32

1

0

32

0

321

V2=2

V2=1

Virtualstate

Virtualstate

1

0Pump

tens of ns

Both processes use the stimulated Raman effect.

If we pump a single level of v2=1 through a Q(J) transition of the fundamental band, and record the intensity of Q(J) transitions of the hot band (v2=2←v2=1) at controlled time delays, (i.e. number of collisions) we can monitor the time evolution of the rotational populations, and obtain the state-to-state coefficients.



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Experimental Setup



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Rate constants: Experimental results

155 K155 K



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Experimental results: the k-matrix

Units: s-1mbar-1;

J=Ji kf←i 1 3 5 7 9 11 13 15 17 19 21

1 -41.61 (2.85)

8.01 (0.13)

3.70 (0.08)

2.38 (0.07)

1.99 (0.06)

1.21 (0.09)

0.60 (0.14)

0.44 (0.22)

0.01 (4.20)

0.01 (7.14)

0.01 (15.4)

3 16.78 (0.28)

-36.74 (1.38)

10.25 (0.09)

5.10 (0.08)

3.58 (0.07)

2.69 (0.08)

2.28 (0.13)

1.82 (0.19)

0.31 (4.26)

0.13 (7.37)

0.01 (15.2)

5 10.00 (0.21)

13.25 (0.12)

-36.22 (1.19)

11.17 (0.06)

5.54 (0.07)

4.53 (0.09)

3.42 (0.13)

2.33 (0.20)

2.40 (4.85)

0.01 (8.38)

0.01 (16.4)

7 6.62

(0.19) 6.78

(0.10) 11.48 (0.06)

-36.76 (1.19)

10.89 (0.10)

7.04 (0.11)

5.24 (0.19)

3.93 (0.20)

1.80 (4.93)

0.92 (8.33)

0.01 (18.0)

9 4.84

(0.16) 4.17

(0.08) 4.99

(0.07) 9.53

(0.09) -34.70 (1.22)

9.63 (0.12)

5.82 (0.16)

4.78 (0.22)

3.08 (4.43)

2.25 (7.36)

0.01 (16.1)

11 2.26

(0.16) 2.40

(0.07) 3.13

(0.06) 4.73

(0.08) 7.38

(0.09) -34.16 (1.14)

7.96 (0.15)

5.93 (0.18)

4.23 (3.06)

1.70 (5.24)

0.01 (11.7)

13 0.77

(0.18) 1.39

(0.08) 1.61

(0.06) 2.40

(0.09) 3.04

(0.08) 5.43

(0.11) -32.01 (1.78)

7.92 (0.22)

5.01 (3.02)

0.01 (6.35)

1.81 (11.9)

15 0.35

(0.17) 0.68

(0.07) 0.67

(0.06) 1.10

(0.06) 1.53

(0.07) 2.47

(0.08) 4.85

(0.13) -31.74 (3.71)

6.91 (4.22)

2.82 (8.28)

0.01 (13.6)

17 0.00

(1.80) 0.06

(0.87) 0.38

(0.77) 0.28

(0.76) 0.54

(0.78) 0.97

(0.70) 1.69

(1.02) 3.81

(2.33) -27.56 (12.3)

0.44 (4.97)

15.91 (9.19)

19 0.00

(1.53) 0.01

(0.75) 0.00

(0.66) 0.07

(0.64) 0.20

(0.65) 0.19

(0.60) 0.00

(1.07) 0.77

(2.28) 0.22

(2.48) -8.27

(21.8) 0.01

(8.98)

J=J f

21 0.00

(1.50) 0.00

(0.70) 0.00

(0.59) 0.00

(0.63) 0.00

(0.64) 0.00

(0.61) 0.14

(0.91) 0.00

(1.70) 3.60

(2.08) 0.00

(4.07) -17.81 (44.1)



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Rate constants: summary

• We have developed a time-resolved pump probe technique for the measurement of state-to-state rotational energy transfer rate constants that uses the stimulated Raman effect both in the pump and probe stages.

• Advantages: simple linear relationship between signal and populations. Our numerical approach accounts for multiple collision events.

• Disadvantages: experimentally complex, limited sensitivity.

• It has allowed the determination of the state-to-state rotational energy transfer coefficients for ortho-acetylene at 155 K WITHOUT resorting to any fitting or scaling law (for J<17).

• We have nevertheless checked how well some of these laws reproduce our results: EG, PEG, IOS-P, ECS.

Doménech, Martínez, Bermejo, JCP 132, 154303 (2010)



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• We have recorded the Q-branch of 2 at 165 K and 298 K• Pressure range: 0.5 – 25 mbar @ 165 K 0.5 – 50 mbar @ 296 K• All the Q-branch is recorded in a single scan ~3 cm-1, step 0.0005 cm-1

• All lines within a scan are simultaneously fitted–The trace @ 0.5 mbar is fitted to a sum of Gaussians with the same width

–γDoppler=0.0036 cm-1 @ 165 K; γDoppler=0.0048 cm-1 @ 298 K;–γILS=0.0039 cm-1

–The rest of traces are fitted to a sum of Voigt profiles with γG fixed to the 0.5 mbar value

• Lorentzian widths, for each J, are fitted to

Linewidths: Experimental approach

2 2 2G Doppler ILS

0,JL L P



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Linewidths: results



17

Linewidths: results



18

Linewidths: results



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Linewidths: results @ room T



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Linewidths: results @165 K



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Summary and future work

• Work is in progress. Excess noise and chirping in the PDA have to be solved.

• Our time-resolved Raman-Raman double resonance set-up can provide state-to-state rotational energy transfer rate constants without resorting to fitting laws.

• The (provisional) broadening coefficients are compatible with the total depopulation rates al low T and with previous measurements at room T.

• Multi-spectrum fits with Galatry or speed-dependent Voigt profiles.• Pure rotational Raman spectra. • Line-mixing predictions.• C2H2 PES (Bartolomei et al. IFF CSIC)• Extension to other systems of atmospheric, astrophysical or

combustion relevance: N2-N2; O2-O2; x-H2.



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Linewidths: results



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Documents

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 1 TIME-RESOLVED ROTATIONAL