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Pulsed-laser photolysis of chromium and molybdenum hexacarbonyls at 308 nm: Simultaneous monitoring of optical and acoustic emissions

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An acoustic technique developed earlier was applied to measure the molar number of fragments produced in the XeCl-laser-induced gas-phase photolysis of Cr(CO)6 and Mo(CO)6. The bluish-green Cr and Mo emissions were also monitored and correlated with the acoustic signal. For laser fluence ranging up to about 25 mJ/cm2 (38 mJ/cm2), the Cr (Mo) optical signal varied with laser fluence asy =dx 3, in agreement with the model that highly excited metal atoms were produced by direct three-photon processes. The acoustic signal varied with fluence asy = ax + bx 2, consistent with the model that most fragments were formed via sequential and direct two-photon processes. Quite expectedly, all coefficientsa,b andd vary linearly with carbonyl partial pressure. Both optical and acoustic signals showed onset of saturation at ≈ 25 mJ/cm2 (38 mJ/cm2) for the case of chromium (molybdenum) hexacarbonyl, suggesting that all photolysis channels were equally affected by the depletion of parent molecules. By assuming a common depletion mechanism, upper limits on the branching ratios of the various photolysis channels were estimated.

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    The value of the absorption cross-section σ at 308 nm was recalculated using the absorbance data of [8] and a Cr(CO)6 vapor pressure of 0.16 Torr. This value of σ is about 1.9 times larger than that given in Ref. [8]. It is noteworthy that the value of σ at 248 nm was also reported in Ref. [13] to be about 1.7 times larger than the corresponding value given in Ref. [8]

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    Our estimate of σ for Mo(CO)6 at 308 nm is about half that given in Ref. [8]. We should also point out that the value of σ at 248 nm reported in Ref. [15] is also about two times smaller than that given in [8]

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Correspondence to N. H. Cheung.

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Chan, C.W., Cheung, N.H. Pulsed-laser photolysis of chromium and molybdenum hexacarbonyls at 308 nm: Simultaneous monitoring of optical and acoustic emissions. Appl. Phys. B 61, 515–523 (1995). https://doi.org/10.1007/BF01081283

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