Why Does Methane Absorb Light Below 274nm If It Photodissociates?

[Aditya Rao]

Hello everyone, I am Aditya. I have just begun my graduate studies at the Hebrew University of Jerusalem. I work on Photochemistry. I hope to connect with more people in PF and learn new things! :)

 

[cnh1995]

Hello everyone, I am Aditya. I have just begun my graduate studies at the Hebrew University of Jerusalem. I work on Photochemistry. I hope to connect with more people in PF and learn new things! :)

Welcome to PF!

 

[EMH121]

HI Aditya:I would like to ask you and the forum a question about atmospheric photodissociation using methane as an example.Methane, CH4, has a photodissociation energy of 439 kJ/mole at 298oK, meaning thatCH4 + hv = CH3 + H , hv<274nmIt also has an “absorption cross section” envelope that starts at ~170nm and increases at lower wavelengths.My puzzlement is as follows:

1) If the CH4 molecule is decomposed at <274nm, why does it have an absorption profile (absorb light) at lower wavelengths (the molecule should no longer be intact, correct?)?

2) If the absorption profile has some kinetic function, why doesn’t the profile start near 274nm?I know that the questions are rooted in my ignorance, but could you please help enlighten me. Thank you in advance.

EMH121