Alt_L is supposed to work as Meta_L (if there is no other Meta
key & Meta_L is not mapped to any other key etc.). - it didn't
in XTerms on my system.
the following entry in ~/.Xresources seems to fix the problem:
!! meta stuff XTerm*eightBitInput: false XTerm*metaSendsEscape: true
gregoa, 2006-01-23