quantum field theory - Tachyon vertex operator (Polchinski's book)

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  I would like to know how does Polchinski in his book "derive" what is the "tachyon vertex operator" (..as say stated in equation 3.6.25, 6.2.11..) I can't locate a "derivation" of the fact that $:e^{ikX}:$ is the tachyon vertex operator.

  
  

  (..I understand that it follows from some application of the state-operator map but I can't put it together..)

  
  


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  And then what is the meaning of the higher vertex operators" - which are of the form of arbitrary number of either operators of the above kind or the derivatives of $X$ w.r.t either $z$ or $\bar{z}$. (..like in equation 6.2.18..)