Optical-Optical Double Resonance Spectroscopy of CH2 1 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..

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

The bond dissociation energy of singlet CH2 is 31797 ± 27 cm-1 The bond dissociation energy of singlet CH2 is 31797 ± 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)

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

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 11892.2 11891.8 11892.0 400 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

The spectra reveal rotational levels in CH2 near 29,000 cm-1 above the lowest singlet level They are regular (B=9.4357 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.

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 116-117 and 147-151.

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

UV saturation-depletion spectroscopy gives another way of detecting higher levels b 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

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.

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 ?