Molecular hydrogen (H2) does not absorb light in the frequencies of the Balmer series of atomic hydrogen. Instead, molecular hydrogen begins to absorb in the vacuum ultraviolet range, with its first absorption occurring around 110 nm. In contrast, atomic hydrogen's first absorption is Lyman alpha at 121.5 nm. While there are molecular states of hydrogen that correlate with the n=2 levels of atomic hydrogen, these transitions require external excitation, such as an electric discharge, to be observed.
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lightarrow said:Is it true, as someone says, that molecular hydrogen in its fundamental state don't (almost?) absorb in the frequencis of the Balmer series of atomic hydrogen emission spectrum?
Thanks.
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lightarrow
Thank you very much, Quantum defect.Quantum Defect said:Molecular hydrogen does not begin to absorb until you get into the vacuum ultraviolet. The hydrogen atom in its ground electronic state (n=1) also does not absorb any light until you get into the vacuum ultraviolet. For the atom, the first absorption is Lyman alpha (121.5 nm). For molecular hydrogen, the first absorption is at even shorter wavelengths (ca. 110 nm).
The Balmer lines of Hydrogen atoms have n=2 as the lower state quantum number. There are hydrogen molecular states that correlate with the n=2 levels, and you do see molecular transitions that have these states as the lower state. You can't see these unless you excite the hydrogen with other means (e.g. an electric discharge).