Research

Research in my group centers on elucidating the electronic structure and chemical bonding of small neutral and cationic metal clusters, semiconductor clusters, and unsaturated metal-ligand complexes via electronic spectroscopy. We employ a wide range of technologies from lasers to mass spectrometers to extract the information we seek. With these tools we work to understand the chemical bonding in these systems and to identify the determinants of molecular structure and reactivity, particularly in the complicated transition metal systems. In grand scope, this work provides detailed and fundamental knowledge about the inner workings of matter on a small scale that can help human kind better understand processes as seemingly diverse as the catalytic production of major industrial materials and the birth of stars.

There are three instruments in my research group, a resonant two-photon ionization (R2PI) spectrometer, a dispersed fluorescence (DF) spectrometer, and a pulsed-field ionization, zero electron kinetic energy (PFI-ZEKE) spectrometer. These three instruments all use the same source to create a molecular beam of reactive molecules, but then interrogate them in complementary ways. Since all three instruments use the same molecular beam source, let me describe it first.

A. Molecular Beam Source

All of our instruments use a pulsed laser to vaporize a metal target. Vaporization involves the deposition of approximately 10 mJ of tightly focused laser energy onto the metal sample in just 10 ns to generate an extremely hot plasma (about 10,000 K). The resultant plasma and all its contents (metal atoms, metal ions, and electrons) are then swept into the vacuum chamber by the high pressure helium (1-10 atm) that is flowing over the metal target at the same moment. In our source chamber, that expansion into vacuum occurs through an orifice the diameter of which (2 mm) is much larger than the mean free path (5 nm) of the gas expanding through it. What this means is that the contents of the expansion undergo numerous collisions upon exiting the high pressure region over the metal sample. This type of expansion is called a supersonic expansion because the ultimate speed attained by the particles in the beam is much greater than the local speed of sound, which drops to very low values as the gas expands and cools. What is important to us is the fact that the large number of collisions upon expansion serve to cool the contents of the beam electronically, vibrationally, and rotationally. Typically, vibrational motions are cooled to 50-100 K and rotational motions to 5 K. It may not be obvious, but this greatly simplifies the spectra we observe. The simplification results from the fact that all the molecules are in just a few different quantum states. The only transitions that can be observed originate from these few populated levels. Some of the molecules we have studied would absorb light everywhere (and their spectra would be uninterpretable) if it weren't for supersonic cooling.

In most of my past work, the expansion gas was helium or argon. In more recent work, however, we have been seeding our expansion with about 3% methane. The extreme conditions of the plasma allow methane to break apart into atoms or fragment radicals that then react with the metal atoms in the plasma. The result of this plasma chemistry has been a dizzying array of new molecules that I describe in the next few sections.

In this instrument, molecules produced in the molecular beam are sent into a time-of-flight (TOF) mass spectrometer that is housed in a high vacuum chamber attached to the source chamber. In the most typical arrangement for this experiment we direct the output of a tunable dye laser down the axis of the molecular beam to excite the molecules, and then expose the molecules (20 ns or so later) to the ultraviolet light from an excimer laser which is directed across the axis of the molecular beam. Though there are many other arrangements that can be used in special cases, this is the one that captures the essence of R2PI spectroscopy. If we expose the molecules to photons from the excimer laser with a short enough wavelength, ions are produced all the time regardless of the wavelength of the dye laser photon. This doesn't give us much physical information about the molecule, but it does help us identify which molecules are in the molecule beam. It also allows me an opportunity to describe the TOF mass spectrometer.

If we use a short wavelength excimer laser photon, a large number of ions are produced every time the laser fires. The ions are produced in a static electrostatic field which accelerates them to a specific kinetic energy. Since the kinetic energy is equal to mv2/2, a correlation between velocity and mass is produced, with lighter ions traveling more rapidly than heavier ones. We detect the ions at the end of a two meter flight tube with a microchannel plate ion detector, and the time between ion production and detection can be converted into mass. By measuring the signal coming out of the detector every 10 ns, we are able to record an entire mass spectrum on each experimental cycle. In this way we can tell what was produced in our source chamber no matter how complicated the plasma reactions are that lead to the observed products.

Once we have identified what molecules are in the molecular beam, we change the wavelength of the excimer laser so that no ions are produced by the excimer laser alone. The key to obtaining an optical spectrum is to use photons with an energy which is less than that required to ionize the molecule. The dye laser pulse is then used to excite the molecule to an excited electronic state. The subsequent excimer laser pulse, with sufficient energy to ionize the excited state but not the ground state, is then fired a few nanoseconds after the first laser. As the first laser is scanned, no ion signal is detected at the mass of interest until a resonant transition is found. When molecules absorb a photon from the first laser beam, exciting them to an excited electronic state, absorption of a photon from the second laser beam then ionizes the molecule. By monitoring the ion signal at a particular mass as the first laser is scanned in wavelength, an absorption spectrum that is specific to that mass may be recorded. Moreover, by simultaneously monitoring the ion signal at several masses, it is possible to record the spectra of several different molecules at the same time. In fact, it is even possible to separately record the spectra of several different isotopic modifications of a given molecule in the same scan.

Given the fact that our source produces a wide variety of chemical species, it is crucial to be able to identify the molecule which is responsible for a given spectroscopic transition. This has proven especially useful in recent studies we have conducted in which chromium is vaporized in a stream of helium containing 3% CH4. In the same general spectral region, we have been able to identify spectroscopic transitions in CrH, CrC, CrCH, CrCH2, CrCH3, CrC2H, and CrC3H2, all with absolute certainty as to the identity of the absorbing molecule. This is a powerful advantage that the R2PI method enjoys compared to other spectroscopic techniques.

The spectra obtained with the R2PI instrument allow one to locate excited electronic states and measure their vibrational frequencies and anharmonicities. When rotationally resolved spectra are obtained it is also possible to deduce bond lengths, bond angles, and often the electronic symmetry of the upper and lower states. It is not generally possible to measure the vibrational levels of the ground electronic state, however, since usually only v=0 and possibly v=1 are populated in the supersonic expansion. It is also difficult to probe excited electronic states lying less than 9000 cm-1 above the ground electronic state, because it is difficult to generate photons with tunable frequencies lower than 9000 cm-1. These limitations led us to construct a second instrument, which builds upon the information obtained from the R2PI instrument.

To complement the capabilities of the R2PI spectrometer we have built a dispersed fluorescence (DF) spectrometer. In this instrument, the molecules entrained in the supersonic expansion are crossed with a tunable dye laser beam, and any fluorescence which is emitted is collected with a system of lenses and sent into a spectrograph. Within the spectrograph, the collected light strikes a diffraction grating and is dispersed into its wavelengths, with different wavelengths being brought to a focus at different positions on a CCD (charge-coupled device) detector array. The CCD allows signals to be integrated over a long period of time and then sent to a computer for storage and analysis.

Once a transition in a molecule of interest has been identified on the R2PI spectrometer, the sample may be brought over to the DF spectrometer so that the fluorescence from the initially pumped level may be examined. Since the molecules are isolated from collisions in a molecular beam, no collisional deactivation is possible, and all fluorescence originates from the initially pumped level. Fluorescence to the various vibrational levels of the ground state occurs at different wavelengths, and measurement of these emitted wavelengths allows the vibrational levels of the ground state to be deduced. Moreover, if low-lying excited electronic states are present, fluorescence to these states allows them to be located and their vibrational levels to be measured as well. Unlike the R2PI, it is not as obvious which molecule gives rise to a particular fluorescence signal, and this is why both machines are crucial to a full understanding of a new molecule.

In a joint effort with Professor Peter Armentrout (also here at Utah), we have built a third instrument, which is dedicated to the collection of spectroscopic data on transition metal-containing cations. Initially, this machine can be thought of as an R2PI spectrometer with added capabilities. Indeed, it is possible to collect R2PI type spectra on this machine, but this would ignore most of its power. What separates this new machine from the R2PI instrument is its ability to ionize molecules with far more finesse than the R2PI and its ability to detect the ejected electrons.

An understanding of the data collected with the PFI-ZEKE apparatus requires some understanding of Rydberg states of atoms and molecules. All neutral atoms and molecules have a series of highly excited electronic states converging to the ground state of the cation. For the highest energy states, the excited electron orbits at long distances and is blissfully unaware of the fact that the underlying cation has structure. The electron is so far away from the underlying cation that all it sees is a +1 point charge. Thus, the highest electronic states are just like the high-lying electronic states of a hydrogen atom, and may be characterized by a principal quantum number, n. These high-lying states are called Rydberg states. Thus, there is a series of Rydberg states converging to the ground vibrational level of the ion, another set converging to v=1, another set converging to v=2, etc. If we could somehow restrict ourselves to measuring the energy of just the highest Rydberg states, then we could map out the energies of the v=0, 1, and 2 vibrational levels of the cation.

The way this experiment is implemented in practice is to use two tunable photons to excite the molecule to a high Rydberg state, very close to the ionization threshold. After a suitable time delay (about 4 microseconds), a weak electric field is turned on. This weak electric field can cause the very highest Rydberg states to ionize, and the ejected electrons are then detected using a microchannel plate detector. By using a pulsed electric field of strength Îµ (in Volts per cm), we are able to restrict the range of Rydberg states which are ionized to those which lie within about 4% (in cm-1) of the ionization limit. The experiment is then to scan the laser across the ionization threshold, monitoring electrons which are ejected by the pulsed field. If Rydberg states are populated which are too low in energy, the weak field cannot ionize them, and no electron signal is detected. If the laser excites the molecule above the ionization limit, an electron is ejected immediately, and this electron has drifted away by the time the electric field is pulsed. Thus, we are sensitive only to electrons which come from the very highest Rydberg states. Since all vibrational levels of the cation have a Rydberg series converging to them, this means that we can in principle see a whole series of sharp peaks corresponding to the different vibrational levels of the cation as we scan higher in energy while looking for electrons detached by the pulsed field.

This experiment, as described, has the potential to collect three different types of data. First, by scanning across the ionization threshold of the molecule it is possible to determine the ionization energy of the neutral. Second, we can collect information about the vibrational structure of the ground state of the cation. Third, it is also possible to collect information about the rotational structure of the cation if the resolution of the instrument is sufficiently high. Rotational levels of the cation also have a series of Rydberg levels of the neutral converging to them so there is essentially no difference between this sort of experiment and the one in which we look at vibrational structure. To date the best resolution we have attained is a FWHM of 1.3 cm-l which is sufficient for rotationally resolved work on some transition metal oxide and carbide cations (MO+ and MC+).

It is possible to determine ionization energies of molecules with the R2PI under favorable circumstances, but the PFI-ZEKE apparatus provides a general method which should be successful in most cases. In combination with cationic bond strengths determined by Peter Armentrout or other researchers we can then determine neutral bond strengths via the thermochemical cycle D0(M-L) = IE(M-L) + D0(M+-L) - IE(M) where M is the metal and L is the ligand bound to that metal. The PFI-ZEKE experiment will also provide vibrational frequencies and rotational constants for the cation, information that has, until recently, only been experimentally available for the neutral species.

The PFI-ZEKE spectrometer is a new instrument in my group, and we are still learning the experimental tricks that are needed to make it work well. In many ways we are still finding out the limitations and the advantages of the method. I am very optimistic that this method will open up a large number of new species for us to study. We have recently published a rotationally resolved study of YO+ in which we have determined the ionization energy of YO to be 49 304.316 " 0.031 cm-1, and the bond length of YO+ to be r0 = 1.7463 " 0.0006. To date there are only about 3 other groups in the world who have applied the PFI-ZEKE method to the types of molecules we plan to study. A vast number of interesting chemical species are waiting to be studied by this technique, and we are now in the position to do so.

Over the past several years we have published the first spectroscopic observations of a large number of molecules. There is nothing I like better than to be able to write up a definitive report of a new molecule which was previously unknown, and to provide details such as the nature of the ground and excited electronic states, including bond lengths, vibrational frequencies, bond dissociation energies, and electronic state symmetries. It is very exciting to be the first person in the world to learn anything about a new molecule, and then to be able to provide a detailed understanding of how the molecule is held together. The following molecules represent species for which our group provided the first spectroscopic identification in the gas phase:

Coinage Group Dimers: CuAg, CuAu, and AgAu
Nickel Group Dimers: NiPd, NiPt, PdPt, and Pt2
Mixed Nickel-Coinage Group Dimers: NiCu, NiAu, and PtCu
Transition Metal Aluminides: AlCa, AlV, AlCr, AlMn, AlCo, AlNi, AlCu, AlZn, and AlY
Mixed Early-Late Transition Metal Dimers: VNi, TiCo, YCo, YNi, ZrCo, ZrNi, NbCo, NbNi, ScNi, YPd, and Ycu
Early Transition Metal Dimers: TiV, Ti2, Zr2, TiZr, TiNb, ZrV, NbCr
Transition Metal Cations: Co2+, Ti2+, V2+, and Co3+
Transition Metal Oxides, Carbides and Silicides: MoO, TiC, VC, CrC, NiC, MoC, PdC, WC and NiSi
Main Group Metal Dimers: LiCa
Coinage Group Trimers: Cu2Ag, Cu2Au, CuAgAu, Ag2Au, and Au3
Main Group Trimers: Al3, Si2N, and Bi3
Semiconductor Dimer: GaAs
Unsaturated Polyatomic Transition Metal-Ligand Complexes: CrCCH, CrCH2, CrCH3, CrC3H2, NiCH3, HfC2

I am particularly excited about these results on unsaturated polyatomic transition metal-ligand complexes, since these species are directly relevant to homogeneous and heterogeneous catalysis and absolutely no gas-phase spectra of species like these have ever been recorded before. In fact, the only gas-phase spectra of polyatomic open d-subshell transition metals to have been reported in the literature to date are of triatomic molecules: TiCH, VCH, NbCH, TaCH, WCH, YC2, YNH, ScNH, YOH, CuOH, NiCl2, CuCl2, YbCCH, and FeCH3. The molecules listed above extend the range of molecules which can be investigated in the gas phase quite substantially, and will provide a great improvement in our understanding of organometallic radicals.

Of course, the identification of a new molecule is distinctly different than providing useful information about that molecule. Below is a list of some of the useful information we have provided for the first time. I have chosen to list just one parameter, the ground state bond lengths. These bond lengths may be given as r0 (corresponding to the average bond length in the v=0 vibrational level) or re (corresponding to the bond length at the minimum of the potential energy curve, obtained by extrapolating the average bond lengths of a series of vibrational levels back to the minimum of the potential curve). Here is a list of the bond lengths first measured in my group:

Other Transition Metal Dimers:

re(CrMo, X 1Î£+) = 1.8182 + 0.0015
re(V2, X 3Î£g-) = 1.77
r0(Ti2, X 3Î”g) = 1.9422 + 0.0008
re(YCu, X 1Î£+) = 2.6197 + 0.0006
r0(VCr, X 2Î”5/2) = 1.7260 + 0.0011
r0(NbCr, X ) =ï¿½ 1.8940 + 0.0003
r0(TiCo, X 2Î£+) = 1.8508 + 0.0004
r0(ZrCo, X 2Î£+) = 1.9883 + 0.0004

Main Group Dimers:
r0(GaAs, X 3Î£-) = 2.53 + 0.02
re(Al2, X 3Î u) = 2.701 + 0.002

Transition Metal Aluminides:
r0(AlCa, X 2Î ) = 3.1479 + 0.0010
r0(AlV, X Î©=0) = 2.620 + 0.004
r0(AlMn, X 5Î ) = 2.6384 + 0.0010
r0(AlCo, X Î©=3) = 2.3833 + 0.0005
r0(AlNi, X 2Î”5/2) = 2.3211 + 0.0007
r0(AlCu, X 1Î£+) = 2.3389 + 0.0004
r0(AlZn, X 2Î ) = 2.6957 + 0.0004
r0(AlY, X 3Î£0-) = 2.8728 + 0.0012

Transition Metal Oxides:
r0(MoO, X 5Î ) = 1.70

Linear Triatomic Molecule:
r0(Si-N-Si, X 2Î 1/2,g) = 1.6395 + 0.0014

Transition Metal Carbides:
r0(NiC, X 1Î£+) = 1.63084 + 0.00010
r0(MoC, X 3Î£-) = 1.68711 + 0.00019
r0(RuC, X 1Î£+) = 1.6081 + 0.0001
r0(PdC, X 1Î£+) = 1.7124 + 0.0008
r0(WC, X 3Î”1) = 1.7143 + 0.0002

Transition Metal Silicides:
r0(NiSi, X 1Î£+) = 2.032 + 0.003
r0(PtSi, X 1Î£+) = 2.0629 + 0.0002

Nickel Group Dimers
Ni2 2.042 + 0.002 eV*
NiPt 2.798 + 0.003 eV
Pt2 3.14ï¿½ + 0.02ï¿½ eV

Early-Late Intermetallics:
TiCo 2.401 + 0.001 eV
VNi 2.100 + 0.001 eV
YCo 2.591 + 0.001 eV
YNi 2.904 + 0.001 eV
ZrCo 3.137 + 0.001 eV
ZrNi 2.861 + 0.001 eV
NbCo 2.729 + 0.001 eV
NbNi 2.780 + 0.001 eV

Transition Metal Aluminides:
AlV 1.489 + 0.010 eV
AlCr 2.272 + 0.009 eV
AlCo 1.844 + 0.002 eV
AlN 2.29ï¿½ + 0.05ï¿½ eV*

Group 4 and 5 Diatomic Metals:
TiV 2.068 + 0.001 eV
TiNb 3.092 + 0.001 eV
V2 2.753 + 0.001 eV*
VZr 2.663 + 0.003 eV
TiZr 2.183 + 0.001 eV
NbCr 3.0263 + 0.0006 eV
Zr2 3.052 + 0.001 eV*

Transition Metal Cations:
Ti2+ 2.435 + 0.002 eV
V2+ 3.140 + 0.002 eV
Co2+ 2.765 + 0.001 eV
Co3+ 2.086 + 0.002 eV
V3+ 2.323 + 0.001 eV
TiO+ 1.763 + 0.001 eV

Other Metal Dimers and Trimers:
Rh2 2.4059 + 0.0005 eV
Al2 2.701 + 0.005 eV

IV. Current Research Funding

1.	National Science Foundation Research Grant CHEâ€‘0415647
	Amount: $446,487
	Grant Period: 1 July 2000 - 30 June 2007
	Title: "Laser Spectroscopy of Gas Phase Metal Clusters"

 2. 	Department of Energy, Basic Energy Sciences Research Grant
	Amount: $410,800
	Grant Period: 1 November 2004 - 31 October 2007
	Title:"Spectroscopy of Organometallic Radicalsï¿½

 VII. Pending Research Proposals
	
	None at present.

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Michael D. Morse

Research Accomplishments, Activities, and Future Directions