1. Optical-Optical Double Resonance Spectroscopy of CH21 A
Zhong Wang, Greg Hall, Trevor Sears and Kaori Kobayashi*
Chemistry Department
Brookhaven National Laboratory and *Toyama University, Japan
June 19th, 2007
62nd Ohio State Symposium on High Resolution Spectroscopy
Thank you Marsha, it’s a pleasure to start off this spectroscopy session at this years meeting. After some thought on what to say today, I decided to tell a story about just one project, rather than give an overview of everything we’ve been doing. That’s mostly covered in the abstract. So this talk is about results from various OODR spectroscopies of just one chemically reactive intermediate involved in combustion chemistry-CH2.
The figures illustrate some of the key results. They show time-dependent absorption profiles for single rotational levels of the radical. Describe the signals in a little detail. Ac stark detection (top right) and saturation depletion, bottom.
Mention the co-workers..

2. The two non-bonding electrons in CH2 can be distributed among two orbitals of similar energy
Increasing energy H 1 B H 3 B 1 H 1 A X a b c
Let’s begin then by briefly looking at pictures representing the electronic configurations for the low-lying states of CH2. There are two non-bonding electrons to place in an in plane and out of plane orbital centered on the carbon atom. The ground state is a triplet….
C.I.
Then higher states 3,1A2, on excitation to σ* (b2 symmetry) orbital. Interested in characterizing the c and higher states
Logo

3. The bond dissociation energy of singlet CH2 is 31797 ± 27 cm-1
The bond dissociation energy of singlet CH2 is ± 27 cm-1 ? from thermochemical tables
But observed uv spectrum ends much below this
1A2
Spectral changes as the dissociation energy is approached?
Additional complexity, lifetime broadening,…
Then we will move on to the observed electronic spectrum of CH2 as we approach the its dissociation threshold. There is a disconnection in the literature between the currently accepted value for the BDE of singlet CH2 and the observed short wavelength termination of the absorption spectrum of the molecule.
The PE curves are cuts through the bending potentials for the low lying singlet states of CH2 …
Can new experiments cast any light on this?
Logo
Yurchenko et al. JMS 208, 136 (2001)

4. Transient absorption of a frequency-modulated c. w
Transient absorption of a frequency-modulated c.w. laser can be used to detect the double resonance
UV reflecting
dichroic
Photolysis
Laser
Bleach laser
Shutter
Si photodiode
amplifiers and
demodulator
Digital Scope
Gas flow cell
RF signal
Generator
CW Probe laser
Phase
Modulator
Ti:sapphire laser
Sample slide from the middle of a presentation. Notice how animation works to allow the speaker to present fairly complex connections in a way that the audience can digest. Notice how the arrows show connections—something that bullets could not achieve.
Reference
This slide is a composite slide from a number of mechanical engineering seniors in ME 4006, a laboratory course at Virginia Tech. These presentations occurred during the Fall 2004 semester.
CH2CO+Ar
308nm excimer laser
tunable ns pulsed laser

5. Fixed pump laser moves population to excited state and transient absorption can be detected from there a b c
Ladder scheme
Pump-Probe
cm-1
400 ns
c ← b
b ← a
Sample slide from the middle of a presentation. Notice how animation works to allow the speaker to present fairly complex connections in a way that the audience can digest. Notice how the arrows show connections—something that bullets could not achieve.
Reference
This slide is a composite slide from a number of mechanical engineering seniors in ME 4006, a laboratory course at Virginia Tech. These presentations occurred during the Fall 2004 semester.
The upper state lifetime is short due to RET and fluorescence

6. The spectra reveal rotational levels in CH2 near 29,000 cm-1 above the lowest singlet level
They are regular (B= cm-1) and show no signs of broadening due to predissociation
We need to look higher in energy, but searching for unknown spectra by scanning the cw laser is time-consuming and restrictive
Sample slide from the middle of a presentation. Notice how animation works to allow the speaker to present fairly complex connections in a way that the audience can digest. Notice how the arrows show connections—something that bullets could not achieve.
Reference
This slide is a composite slide from a number of mechanical engineering seniors in ME 4006, a laboratory course at Virginia Tech. These presentations occurred during the Fall 2004 semester.

7. If we pump a transition between unpopulated levels, the probe laser may still see an effect
Pump field causes broadening and shifting of |b> and |c> due to ac Stark effect. Dressed state picture: a b c a
|b_d>
|c_d>
Sample body slide from the second section of the presentation’s middle. For the first body slide of this second section, consider repeating the corresponding image from the mapping slide. Use the headline (no more than two lines) to say something about this topic. In the body of the slide, support that headline with images and parallel points (no more than four). See CSP, pages and

8. ac Stark effect detected OODR has a different time dependent signature from pump-probe
Probe laser has insufficient power to cause significant population transfer
But the intense pump laser pulse causes a shift and broadening in level b that's detected as a transient decrease in absorption of the probe laser
Sample slide from the middle of a presentation. Notice how animation works to allow the speaker to present fairly complex connections in a way that the audience can digest. Notice how the arrows show connections—something that bullets could not achieve.
Reference
Håvard Holmås, “Pseudospectral Numerical Methods for Modelling of Waves in Pipe Flow,” presentation (Kjeller, Norway: Institute for Energy Technology, 27 September 2006).
Can acquire spectra rapidly by scanning the ns pulsed laser

9. UV saturation-depletion spectroscopy gives another way of detecting higher levelsb c
NIR probe monitors single rotational level, uv pump transfers population on resonance
Observed time-dependent absorption signals
Now we don’t see depletion as we expected, the CH2 absorption signal increases. How is this happening?
Extra CH2 produced by doubled dye laser-requires hot CH2CO here

10. Double resonance spectra nearer CH2 dissociation threshold show many ‘extra’ lines…
Spectra monitoring 110 and 212 as UV laser is scanned near 30,000 cm-1;converted to upper state energy scale
From 110 can go to 101, 221. From 212 go to these plus 303, 321.

11. This may explain the apparent disappearance of absorption lines before the dissociation limit
High level density as we approach dissociation causes loss of intensity for individual lines. No evidence for broadening (yet).
C(3P)+H2
Future: follow spectra to higher frequency and look for broadening or complete loss of signal  upper limit on dissociation energy for CH2 ? Any role for triplet?
Questions ?