Keep character from jumping more than once?

Hey Unity community!

I am working on a racing game, and am finalizing the drifting mechanic. I want to make the drifting work like it does in Mario Kart, in which holding the button produces a small hop, then initiates the drifting physics. I have already got the drifting physics working, but I am having an issue with the small hop. Currently, holding down the drift button produces the hop, but the vehicle continually hops until the button is let go. How do I set up my script so that the vehicle will only hop once per hold?

Here is the jump script that needs to be changed:

var speed : float = 10.0f;
var jumpForce : float = 1000.0f;
var airModifier : float = 0.2f;
var grounded : boolean;

// Update is called once per frame
function Update () 
{
	if (Physics.Raycast(transform.position, -transform.up, 2)) 
		{
			grounded = true;
		} 
	else 
		{
			grounded = false;
		}
//jump if the user pressing button AND our character is grounded
	if (DriftButton.playerJump && grounded)
		{
		rigidbody.AddRelativeForce(transform.up * jumpForce, ForceMode.Impulse);
		}
}

Any help would be great!

Add a small wait time condition:

var speed : float = 10.0f;
var jumpForce : float = 1000.0f;
var airModifier : float = 0.2f;
var grounded : boolean;
var jumpWaitTime : float = 1.0;

private var nextJumpTime : float;

// Update is called once per frame
function Update () 
{
    if (Physics.Raycast(transform.position, -transform.up, 2)) 
       {
         grounded = true;
       } 
    else 
       {
         grounded = false;
       }
//jump if the user pressing button AND our character is grounded
    if (DriftButton.playerJump && grounded && Time.time >= nextJumpTime)
       {
       rigidbody.AddRelativeForce(transform.up * jumpForce, ForceMode.Impulse);
       nextJumpTime = Time.time + jumpWaitTime;
       }
}

I’m working on a drift for a mario kart clone, could you help me out with the drift code?