Research conducted in the laboratory
of
Prof. Timothy Steimle


Generation, Detection, and Characterization of Gas Phase Transition Metal Containing Molecules

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:

    1. 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)
    2. The generation and detection of new polyatomic molecules containing a third row group VIIB and VIIIB atom (Re, Os, Ir and Pt).
    3. 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.

  1. Optical spectroscopy (laser induced fluorescence) using both a cw linear and a pulsed dye laser.
  2. 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:

a) Transition metal nitrides, CrN, VN and ReN

Figure 3. The observed and calculated reduced dipole moment (m /Re) of metal oxides and nitrides including our recent results for CrN and VN.
 
 

The experimental values of the dipole moments for most of the first row transition metal oxides are now known and exhibit the trend given in Figure 3. We recently extended these measurements to the mononitides CrN and VN. Notice that the oxides and nitrides exhibit a significantly different behavior and that the experimental values are consistently lower then the most sophisticated calculations predict. This work has been accepted for publication in JCP. We have also recently recorded to optical stark spectrum of ReN.
 
 

 

b) Platinum polyatomic compounds

Figure 4. The low and high-resolution LIF spectra of what we believe to be PtC2H.

We previously reported recording the low resolution LIF spectrum of what we suspected to be a Pt containing polyatomic molecule generated in the reaction of Pt with CH4. We recently managed to record the high-resolution LIF spectrum (Figure 4) which confirms our suspicion. We are implementing an optical filtering scheme and further cooling that will enable us to in simplify the spectra. We have also recorded the TOF mass spectra for our Pt + CH4 reaction ( Figure 5).
 

Figure 5. The time-of-flight mass spectrum of the reaction products of the Pt ablation with CH4. The carrier of the LIF spectrum of Figure 4 is proposed to be PtC2H4.



 

c) Metal dicarbides

Although the simple metal dicarbides are postulated to exist in the process of metal-catalyzed growth of single walled nanotubes and as building blocks of "met-cars" our recently reported low-resolution study of YC2( JCP 106 2060, 1997) was the first report of their detection. Interestingly, the asymmetric vibrational mode of this "T-shaped" molecule is relatively floppy which is in accordance with what would be expected: the C2 moiety is weakly bound to the metal atom and predisposed to deposition on the nanotube wall. We have recently recorded the high-resolution LIF spectrum of YC2 and are in the process of analysis. Initial analysis indicates that the electronic states are quartets rather than doublets as would be expected based upon a comparison with other isovalent Y containing molecules. We are investigating whether the higher multiplicity may be expected to enhance the catalytic action.

It is postulated that the metal dicarbides associated with "met-car’ production (e.g. TiC2 ) will have a structure quite different from "T-shaped" structure of YC2. Specifically, these molecules should have a relatively short metal-CC bond and a relatively long C-C bond (i.e have the shape of SO2). As proposed, it is our goal to determine if the metal dicarbides can be separated into two classes (i.e. "T-shaped" and "SO2 shaped") and their chemistry correlated to their structure. The first step in these experiments has been to record the TOF spectra of the reaction products of Ti, La and Nb with CH4 . The TOF spectrum for Ti/CH4 is presented in Figure 5. Note that TiC2 is dominant amongst the molecular species. Having now optimized the conditions for the production of TiC2 we will begin a search for the optical LIF spectrum.

Figure 6. The TOF mass spectrum for Ti/CH4 reaction products. Note the dominance of TiC2.

 

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 (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 (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.

 

Panel_3                                                              Panel_4

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