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Ozone is easily produced by Hg lamps with <200nm power (peak@185nm), but O2 also strongly absorbs around 10um. I've poked around but can't find any discussion of ozone production at 10um. Is the photon energy too low at that wl to break up O2?

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    $\begingroup$ Yes, 10 um energy is too low to make ozone. $\endgroup$
    – ACR
    Commented May 18 at 2:12

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Absorption in low energy thermal IR region does not break chemical bonds, but affects vibration of chemical bonds and rotation of molecules.

Energy of $\pu{10 \mu m}$ photons is about 40 times smaller than the energy needed to break the bond of $\ce{O2}$ to form atomic oxygen and then ozone.

There is the wavelength threshold $\lambda = \pu{242 nm}$ to break oxygen molecules to form ozone. Photons of radiation with longer wavelength do not have enough energy to do so.

Note that the requirement of the sufficient photon energy is the mandatory but not sufficient condition. The other mandatory but not sufficient condition is the photon absorption. Even a high energy gamma photon cannot break the bond in $\ce{O2}$ if it is not absorbed (and reemitted with lower energy and/or passing energy to formed fragments).

See the attached absorption chart from Researchgate - Spectral characteristics of the absorbance cross-sections of oxygen and -ozone enter image description here

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  • $\begingroup$ If you had coherent light, via an IR laser, could it reach the required energy (in principle) $\endgroup$ Commented May 19 at 3:04
  • $\begingroup$ Well, you could always achieve thermal dissociation. $\endgroup$
    – Poutnik
    Commented May 19 at 4:55
  • $\begingroup$ O2 does not absorb at 242nm. the energy must be absorbed. $\endgroup$
    – jimchmst
    Commented May 19 at 20:50
  • $\begingroup$ the answer has been updated. From the chart it is obvious O2 starts absorbing at about 242 nm, but very little. $\endgroup$
    – Poutnik
    Commented May 20 at 9:28
  • $\begingroup$ Pulsed CO2 lasers (1 MW peak) can cause dissociation by multiphoton absorption and climbing the vibrational/rotational ladder as it were, until dissociation. The process is complex but involves the huge electric field of radiation bringing levels into resonance for absorption. This method has been used for isotope separation of gasses. $\endgroup$
    – porphyrin
    Commented May 20 at 10:50

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