Broida oven

Molecule sources may be divided into three types thermal, supersonic jet, and cold collisions. Thermal sources (King furnace King, 1926 Broida oven West, et al, 1975 electric or microwave discharge Fehsenfeld, et al., 1965 static gas cell) are simple, convenient, and versatile. However, when the rotational partition function, kT/B, approaches 103, each vibrational band will contain many hundreds of rotational lines, the spectrum will become congested, vibrational bands will overlap each other, and analysis will be difficult. 

The key discovery that opened up the field was made by Harris and co-workers [32-35] in the early 1980s. They found that alkaline earth monohydroxides and monoamides could be made readily in a flow reactor called a Broida oven [36]. The Broida oven is a relatively cool ( 500K), low-pressure ( 5 torr) source of high-temperature molecules that is suitable for spectroscopic studies. The work of Harris and our own work owes a great debt to the pioneering efforts of Broida in developing the source and demonstrating its potential. 

The molecular species in a Broida oven can often be detected through their chemiluminescent emission [32], It is particularly convenient to monitor this emission in the early stages of a low-resolution analysis. The information that can be extracted from a chemiluminescent spectrum recorded with a monochromator is, however, limited. More typically, the molecules are detected by laser-induced fluorescence using either pulsed or continuous wave (CW) dye lasers. 

The molecules in a Broida oven are produced by the reaction of a metal vapor with an appropriate oxidant (Fig. 1). The metal (Mg, Ca, Sr, or Ba) is vaporized in a resistively heated crucible and entrained in a flow of Ar gas. The oxidant is added at the top through an oxidant ring. The reaction of the metal with an oxidizer, for example,... 

Figure 1. The baseplate for the Broida oven metal flow reactor. This baseplate is attached to the bottom of a vacuum chamber (Fig. 2). (a) chemiluminescent flame (b) oxidant injection ring (c) tungsten wire heating basket (d) Ar carrier gas inlet (e) cooling water (f) oxidant gas inlet (g) electrical feeds for heating (h) alkaline earth metal (i) alumina crucible and (j) alumina heat shield. [Reprinted with permission from ref. 28. Copyright 1991 American Association for the Advancement of Science.]...

Exploiting the enhanced reactivity of Ca often requires the use of two tunable dye lasers (Fig. 2), one to excite the metal atoms to the metastable 3F1 state and the second to detect the product molecules. (Excited metal atoms can also be produced in an electrical discharge or in a laser-vaporized plume.) The two laser beams are introduced into the Broida oven chamber... 

One of the recent developments has been the use of Broida oven technology in recording millimeter wave pure rotational spectra (Fig. 3). The pure rotational transitions are recorded in absorption using a free space cell. There are two main groups working in this area, the Ziurys group at Arizona State University [37] and the Saito group [25] at the Institute for Molecular Science in Japan. 

The Broida oven is a nearly ideal source for the spectroscopy of diatomic molecules and small polyatomic molecules such as CaOH. For larger species, however, the spectral congestion is too severe and the collisional relaxation rates are too high to record resonant, rotationally resolved spectra. The solution to this problem is to lower the temperature to eliminate the spectral congestion and to lower the pressure to eliminate the... 

Although we cannot directly detect HCaOH because it probably has a dissociative UV spectrum [14], we can detect another predicted reaction intermediate in some of our experiments. Mechanism A predicts that the CaH molecule will be present in the Broida oven, and with some oxidants we have detected it by laser-induced fluorescence. The CaH molecule is seen when carboxylic acids such as formic acid are used to make the monocar-boxylates such as Sr02CH [42]. Curiously, CaH is not detected [41] when water or alcohols such as CH3OH are used to make alkoxides such as CaOCH3. More experimental and theoretical work is necessary to establish the chemical mechanisms involved in the reactivity of the alkaline earth atoms. 

There have been several studies of the reaction dynamics of the ground and excited states of the alkaline earth atoms with various oxygen-containing molecules under single collision conditions. Although these studies are not directly applicable to the multiple collision regime in the Broida oven, they clarify the dynamics of a single encounter between a metal atom and an oxidant molecule. Oberlander and Parson [43] looked at the reactions of Ca and Sr with water, alcohols, and peroxides. Similar studies... 

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

Isoelectronic analogies are very helpful in understanding the geometry and electronic structure of the monovalent alkaline earth derivatives. For example, formamide, HC(0)NH2, differs from formic acid, HC(0)0H, by the substitution of an NH2 group for OH. The use of formamide as an oxidant in a Broida oven gives rise to a spectrum [93] with three electronic transitions A X, B-X, and C-X (Fig. 20) very similar to the corresponding monoformate derivatives. 

The CaN3 and SrN3 molecules were made in a Broida oven by the reaction of Ca and Sr vapors with hydrazoic acid, HN3 [96], The reaction is relatively vigorous and, in contrast to the other Ca and Sr reactions, exciting the atomic state of the metal increases the reactivity only slightly. The B2 L + -X2 t + and A2Tl-X2I,+ transitions could be identified... 

The excited states of CaNC and SrNC are not well understood although high-resolution spectra have been recorded in the ultracold environment of a molecular beam. The low-lying excited states of CaNC and SrNC should be A2II and B2X+ analogous to CaF (Fig. 7). However, additional unassigned broad red emission is seen in both the Broida oven [102] and molecular beam experiments [39]. 

The sulfur analogues of the alkaline earth monoalkoxides have also been made in a Broida oven by the reactions of Ca and Sr with thiols [111]. 

The preparation and low-resolution spectra of a few calcium and strontium alkylamides were reported in 1987 [118]. By replacing NH3 with monoal-kylamines, NH2R [R = —CH3, —C2H5, —CH(CH3)2, and —C(CH3)3] in a Broida oven, the laser-induced fluorescence spectra of the A-X, B-X, and C X transitions (see for example, Fig. 29) were recorded. There was a strong similarity with the spectra of CaNH2 (Fig. 7) although the symmetry is lower than C2l, for the monoalkylamide derivatives. 

The monoacetylide derivatives are also important examples of simple organometallic molecules. The reactions of excited Ca and Sr with acetylene in a Broida oven resulted in the detection of the CaCCH and SrCCH molecules [127], Table 8 compares the observed and calculated X2E 1 vibrational frequencies. This work was rapidly followed by a rotational analysis of a vibrational band in the A2U X2E+ transition of CaCCH [128], It was assumed that we had found the 0-0 band, but the low-resolution molecular beam experiment of Whitham et al. [39] showed that we had analyzed a hot band. 

Recently, several bands of the A2TI X2E+ transition of CaCCH were analyzed, first in a molecular beam source [129] and then in a Broida oven [130]. The pure rotational spectra of MgCCH [131], CaCCH [132], and SrCCH [133] are also available. If the C—H and C C bond lengths are fixed to 1.506 and 1.204 A, then the Ca—C bond length is 2.349 A, identical to the CaCH3 value [132], An ab initio calculation by Chan and Hamilton... 

The alkaline earth metals form a host of unique monovalent free radicals. Most of these molecules can be formed by the laser-driven chemical reactions of metal vapors with a wide variety of organic and inorganic molecules. This photochemical production of new molecules has led to an extensive gas-phase inorganic chemistry and spectroscopy of alkaline earth derivatives. In recent years, the Broida oven source has been displaced by the pulsed molecular beam spectrometer. The chemical dynamics and photochemistry of these new molecules are still at a very early stage of investigation. 

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