Research
conducted in the laboratory
of
Prof. Timothy Steimle
Generation, Detection, and Characterization
of Gas Phase Transition Metal Containing Molecules
- Research Scope, Objective and Perspective
The goal of our research is to
investigate the high resolution spectroscopy of transition metal containing
molecules. Our approach is to generate these gas-phase metal containing
molecules using a laser ablation source and extract the data from an analysis
of optical and microwave spectra. Examples of experiments currently being
performed are:
-
A comparative study
between the permanent electric dipole moments of the first row mononitrides VN, CrN, and the third row mononitrides
ReN (previously we measured dipole moments
for MoN, IrN, PtN and TiN)
The generation and
detection of new polyatomic molecules containing a third row group VIIB
and VIIIB atom (Re, Os, Ir and Pt).
The detection and
characterization of transition metal dicarbides
which are proposed to be intermediates in the metal-catalyze growth of
single wall nanotubes (e.g. YC2, LaC2,
and NbC2)and those proposed to be the fundamental building
blocks of "met-cars"(e.g. TiC2).
Experimental Setup
In all our experiments we produce gas phase transition metal-containing
molecules using a laser ablation/supersonic jet source, see figure 1.
Figure 1: The laser ablation/
supersonic jet source.
In this source a solid metal rod is ablated using a Nd:YAG laser in the throat of a pulsed supersonic
expansion of Ar containing a small amount (5-10%) of
a reagent. In the plasma that results from the laser ablation, reactions occur
between the metal and the reagent. The products of which are cooled by the
expansion to temperatures of a few Kelvin. The spectroscopy of the molecules
produced is then studied using Laser Induced Fluorescence and TOF- mass
spectroscopy.
Low
resolution spectroscopy; the detection of new
molecules.
In our low resolution lab we utilize two methods for the identification and
characterization of new species.
- Optical spectroscopy (laser
induced fluorescence) using both a cw linear and
a pulsed dye laser.
- A newly constructed Time Of Flight mass
spectrometer to perform both non-resonant mass analysis and REMPI spectroscopy.
High
resolution spectroscopy.
Some recent work from our lab:
- Electric dipole moments of transition metal monocarbides
Observed and predicted permanent electric dipole moments of MoC (Debye)
|
Exp. |
Ab initio |
DFT |
|
|
All-elec.
MRCI |
ECP.
MRCI |
SVWN |
Bp86 |
Meta-Bp86 |
Lb94 |
B3LYP |
B3LYP |
X3S- |
6.07(2) |
6.15a |
5.87b |
5.218c |
5.209c |
5.200c |
5.080c |
3.145c |
5.50d |
[18.6]3П1 |
2.68(18) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
The molecular orbital correlation diagram used to explain trends in dipole moments for the 4d- metal monocarbides.
Transient
Frequency Modulation Spectroscopy
1. Motivation
Transient frequency modulation (TFM) spectroscopy is a high-resolution, absorption based, technique. In this technique side bands shifted by rf frequencies are put on the laser frequency (see Panel 1) and any imbalance in the absorption or dispersion between the two produces a small rf signal which is detected using a “homodyne” detection technique. Normal frequency modulation (FM) spectroscopy [1,2], has a theoretical minimal detectable absorbency of ~1´10-7 but in reality absorbency of only ~1´10-3 is achieved [1] due to non-resonant “residual amplitude modulation” (RAM). TFM spectroscopy is considerably more sensitive than normal FM spectroscopy. In TFM spectroscopy the FM signal is recorded just prior to, and then during, the presence of the absorbing species (see Panel 2). The two signals are subtracted, canceling all the extraneous frequency dependent absorption or dispersion not associated with the molecules. The enhanced sensitivity for TFM was demonstrated in a joint effort between Prof. Field’s group at MIT and the group (G. Hall and T. Sears) at Brookhaven National Laboratory [3].We previously demonstrated its application to laser ablation sources [4].
Panel 1
Panel 2
References
1.
“Frequency Modulation (FM) Spectroscopy”
G.C. Bjorklund, M.D. Levenson,
W. Length, and C. Ortiz, Appl. Phys. B. 32, 145-152,
(1983).
2.
“Frequency modulation and wavelength modulation spectroscopies : comparison of experimental methods using
lead-salt diode lasers” ,D. Silver, A.C. Stanton and J. A. Silver, Appl. Opt. 31, 718-730 (1992).
3. “Time-resolved frequency modulation spectroscopy
of photochemical transients” J.C. Bloch, R.W. Field, G.E. Hall, and T.J.
Sears, J. Chem. Phys. 101, 1717-1720 (1994).
4. “Transient Frequency Modulation Absorption
Spectroscopy of Molecules Produced in a Laser Ablation Supersonic Expansion
Source,”T.C. Steimle,
M. L. Costen, G.E. Hall, and T. J. Sears, Chem. Phys.
>Lett, 319,
363-367 (2000).
2. Observations at ASU
We are in
the initial stages of making this technique work in our laboratory. The transient signal from the output of the
single side-band mixer when the laser is tuned to the F'=3 ¬ F"=4 component of the QQ14+QR14(0) spectral feature of the A2P1/2-X2S+ band system of LaO at 12635.750 cm-1 is presented in Panel 3. A
conical nozzle was employed. The
approximate 20 msec width of the signal is a
reflection of the transit time of the slug of molecules through the cw-laser beam.
A
comparison of the simultaneously recorded LIF spectrum (taken behind a skimmer
at approximately 0.6m from the source) and the TFM spectrum (taken at 4cm from
the source) is given in Panel 4. In the upper trace TFM spectrum, the phase
between the local oscillator and input to the mixer was altered to show the
effect on the line shape as the balance between the absorption and dispersion
contribution to the output is altered.
3. Future Modifications
The
resolution of the TFM signal can be significantly improved over that in Panel 4 at a sacrifice
to S/N by narrowing the "Box Car" gate and setting it to capture
early arriving molecules [4]. The
sensitivity will be improved by using a slit nozzle and a multipass
White cell.
Mid-IR Difference Frequency Spectrometer
with Stark Modulated
A very sensitive and selective mid-infrared spectroscopic technique is required to detect transient metal containing radical molecules. Broadly tunable, intense, monochromatic lasers (i.e. the equivalent to a cw-dye laser in the visible spectral region) are not currently available in the mid-infrared and the proposed spectrometer (Figure 1) employs a difference frequency generation (DFG) scheme. (A review of mid-IR laser technology can be found in F.K.Tittel, et al Topics Appl. Phys. 2003, 89, 445-516). The saline features of our spectrometer are:
- Monochromatic excitation generated by difference frequency laser techniques for high spectral resolution;
- Slit nozzle, seeded, supersonic expansion source for generation of “cold” molecules ( T < 20 K);
- Balanced dual beam detection for noise reduction; and
- Stark modulation for enhanced selectivity and improved sensitivity.
The difference frequency spectrometer shares many similarities to the ones used by the Nesbitt group (JILA) and the Curl group (Rice University) but with two critical modifications: a) laser ablation, supersonic free-jet expansion for molecular production and b) Stark modulation. The DFG scheme that we employ incorporates two recent technological advances making the method more practical: a) The traditional Ar+ laser is replaced by a powerful compact and low-noise, single frequency, cw, diode-pumped Nd:YAG laser operating at 1064 nm; b) the mixing media is a periodically poled lithium niobate (PPLN) crystal. The periodic-poling facilitates phase matching and eliminates the need for accurate 
temperature control. Our current crystal is uncoated to minimize damaging the surface, has ten channels and is 5 cm long with total coverage of approximately 570 cm-1. The typical output power is 300 mW when pumped with 1W of 1064nm and 300 mW of cw-Ti:sapphire radiation. The monochromatic nature (DnL <1 MHz) of the Nd:YAG and cw-Ti:sapphire are transferred into the tunable mid-IR radiation. Optimizing the sensitivity of the spectrometer is essential for the detection of the insertion products. The relatively high power of the tunable IR radiation makes it possible to implement a dual beam optical balancing scheme for common mode noise subtraction The pulsed nature of the expansion (typical width of 250 ms) allows for a second signal to noise (S/N) enhancement by use of a 3 kHz high-pass filter after the differential amplifier. Both of these S/N schemes are routinely used by the Rice and JILA groups. In a similar vein, we have implemented a Stark modulation/phase sensitive detection scheme similar to that traditionally used in microwave and millimeter-wave spectroscopy. This approach not only greatly enhancing the sensitivity of the spectrometer, but also bias our detection towards the polar molecules that rapidly Stark tune.
Fig. 1 The proposed high-resolution, mid-IR spectrometer to be used in the study of XMCH3 (X=H or halogen atom, M=Rh, Pd, Ir and Pt) insertion products. Cold sample of molecules will be generated in slit nozzle supersonic expansion. The monochromatic IR radiation is generated periodically poled lithium niobate (PPLN) crystal. Dual balanced beam detection, high pass filtering, and Stark modulation/phase sensitive detection will be used to enhance sensitivity and spectral simplification. The current spectrometer does not have the Stark modulation or laser ablation modifications
Preliminary results
Fig. 2 Difference frequency spectra. The absorption spectrum of a seeded molecular beam sample of CH3OH without Stark modulation (upper trace) and with Stark modulation (lower trace). Stark modulation improves the S/N and simplifies the spectrum. Only the 10,1-11,0 spectral feature rapidly Stark tunes. The line width of approximately 80 MHz (FWHM) is due to residual Doppler broadening. An estimated rotational temperature of approximately 10 K is obtained from the relative intensity.
Return to Steimle Home Page
